Greetings,
No, the books show no "notable spy networks or systems" and certainly not a Caste of Spies.
The next closest thing to a systematic group of spies was in Corcyrus. These spied both on Argentum and the population of Corcyrus itself. It is also presumed that Argentum had spies in Corcyrus. But the two cites were at war. So instead of these spies being part of a wider network, they were instead gaining information to help in the war effort. You can read more in the context surrounding Kajira of Gor, Page 176.
Others keep private networks of spies, Samos of Port Kar and Rask of Treve to name a couple. (Raiders of Gor, Page 191 and Captive of Gor, Page 298) But again, these do not constitute what you are describing.
Following now are just some examples of spies mentioned through the series.
"The women of the Walled Gardens know whatever happens on Gor," she replied, and I sensed the intrigue, the spying and treachery that must ferment within the gardens. "I forced my slave girls to lie with soldiers, with merchants and builders, physicians and scribes," she said, "and I found out a great deal." I was dismayed at this - the cool, calculating exploitation of her girls by the daughter of the Ubar, merely to gain information.
Tarnsman of Gor    Book 1    Page 108
"What is your business in Tharna?" he asked.
"I should like to obtain a tarn," I said, "for a journey I have in mind." I had answered him rather directly. I assumed him to be a spy, charged with learning my reasons for visiting Tharna. I scorned to conceal this reason, though the object of my journey I reserved to myself. That I was determined to reach the Sardar Mountains he need not know. That I had business with the Priest-Kings was not his concern.
Outlaw of Gor    Book 2    Pages 70 - 71
"Then," I said to the Muls who rode with me, "you are spying on me for Sarm."
"Yes," said Mul-Al-Ka.
Priest-Kings of Gor    Book 3    Page 141
I was horrified at what had been done and I wondered what the sensations would be or even if I would be aware of what I was doing when under the control of Priest-Kings. But most I feared how I might now be used to injure Misk and my friends. I might be sent back among them to spy, to foil their plans, perhaps even to destroy, perhaps even to slay Misk, Al-Ka and Ba-Ta and other leaders, my friends all.
Priest-Kings of Gor    Book 3    Page 254
"You could send Implanted Ones to spy," I suggested.
"There are no more Implanted Ones," said Misk. "We have recalled them and are removing the control nets. They may return to their cities or remain in the Nest, as they please."
Priest-Kings of Gor    Book 3    Page 313
These animals, trained prairie sleen, move rapidly and silently, attacking upon no other provocation than trespass on what they have decided is their territory. They respond only to the voice of their master, and when he is killed or dies, his animals are slain and eaten.
There would be no question of night spying on the Wagon Peoples.
Nomads of Gor    Book 4    Page 9
"Why have you come to spy?" he asked.
"I am not a spy," I said.
Nomads of Gor    Book 4    Page 19
Cautiously I poked my head over the edge of the roof. The street was deserted save for a girl, who was standing below, looking up toward the roof. She was dressed in veil and Robes of Concealment. It was she whom I had seen before, when I had thought I might be followed. It was she who had inadvertently detained my pursuers. She carried a broken market basket.
"You make a very poor spy, Tarl Cabot," she said.
"Dina of Turia!" I cried.
Nomads of Gor    Book 4    Page 237
"Have you discovered the contact points," I asked, "from which they hope to extend their influence in the cities?"
"Only one seems clear," said Misk. "And we do not wish to destroy it immediately. Such would indicate that we are aware of their plan. Further, innocent rational creatures might be destroyed. Further, if we destroy it, and it is a portion of a network, we will have lost valuable information on the degree of their dispersement and penetration."
"You need a spy, Misk."
Assassin of Gor    Book 5    Page 70
Cernus laughed. "And so we arranged to have you arrive in our house, the trusted spy and agent of Priest-Kings, who would thus think themselves moving secretly and intelligently against us. And here, while we have through the months advanced our cause you have stood by, patiently and cooperatively, a dupe and a fool, our guarantee that Priest-Kings would not send another."
Assassin of Gor    Book 5    Page 279
Another lifted his crossbow, leveling the weapon at my breast. "He is perhaps a spy of the Yellows," said he. "Perhaps," agreed the leader of the crossbowmen.
"How do we know you be Gladius of Cos?" asked another.
I smiled. "The tarn will know me," I said.
"The tarn has tasted blood," said the leader. "It has killed. It feeds. Do not approach the tarn now or it will mean your death."
"We have little time to waste," I said.
"Wait!" cried the leader of the crossbowmen.
I stepped toward the great black tarn. It was at the foot of its perch. It was chained by one foot. The run of the chain was perhaps twenty-five feet. I approached slowly, holding my hands open, saying nothing. It eyed me.
"The bird does not know him," said one of the men, he who had suggested I might be a spy of the yellows.
Assassin of Gor    Book 5    Page 352
"He is a spy," said one of the other men present, who stood beside Ho-Hak. This man was tall, and strong, looking. He carried a marsh spear. On his forehead there was tied a headband formed of the pearls of the Vosk sorp.
I wondered what in particular there might be to spy about on the rence islands.
Raiders of Gor    Book 6    Page 18
"The time is opportune," said Samos, "to offer peace to Cos and Tyros. For one thing, the Council has newly come to power. For another, I have learned from spies that the Ubar of Tyros visits Cos."
Raiders of Gor    Book 6    Page 162
I put spies in Tyros and Cos, and in many of the other ports of Thassa.
Raiders of Gor    Book 6    Page 191
"Those men," said Ena, "are Raf and Pron, huntsmen of Treve, though they range widely in their huntings, even to the northern forests. By order of Rask of Treve by their skill in weapons and their mastery of the techniques and lore of the hunt, and pretending to be of Minus, a village under the hegemony of Ar, made petition, and successfully so, to participate in the retinue of the great Ubar." She smiled at me. "Treve," she said, "has spies in many places."
Captive of Gor    Book 7    Page 298
He approached, pushing two captives before him. They wore the skins of panther girls. Their hands were tied behind their backs. They were fastened together by a single branch, tied behind their necks.
I did not recognize them.
"They were spying," said he.
"No," said one. "We were looking for Verna."
Hunters of Gor    Book 8    Page 317
"I was not always of Tor," I told him. "Originally I was from the north."
"He is a Kavar spy," said one of the lieutenants, at the side of the captain.
Tribesmen of Gor    Book 10    Page 84
"Courtly fool," he said. "Investigating her further, understanding she accompanied you to the Sardar, with the last egg of Priest-Kings, we looked for further connections. Soon it became clear that she had been your confederate, spying for you, in contriving the downfall of the house of Cernus, one of our ablest operatives."
Tribesmen of Gor    Book 10    Page 127
I pointed to the powerful, bloodied captive.
"He is Clitus Vitellius of Ar," I cried. "He is a captain of Ar!"
"A spy!" cried a man.
"Kill the spy!" cried another.
"He says he is Tij Rejar, an oarsman of Tyros, but he is of Ar, of the Warriors! He is Clitus Vitellius! He is of Ar! He is a captain!"
Slave Girl of Gor    Book 11    Page 350
"Very well," said he. "It is true that I enlisted the services of a lowly bond girl in his house, to obtain material from which I might give scent to the sleen."
"She is a spy there?" she asked.
"No," he said, "I tricked her. I used her as a mere dupe in my scheme. It was not difficult. She was only a woman."
Beasts of Gor    Book 12    Page 151
"Did you spy upon us once," I asked, "further down the river?"
"Yes," she said. "It was I. We then determined to try and trap you, for slaves." Ayari, then, long ago, had, as I had suspected, seen a taluna in the forest. He had thought it might have been Janice, gathering wood.
Explorers of Gor    Book 13    Page 411
Once I had thought they might be following me, men put upon my trail by my mistress to spy upon me, to see if I were discharging her errands perfectly, or if my eyes might stray to the ankles of slave girls, but then, as they had turned aside, it had seemed to me that I must be mistaken.
Fighting Slave of Gor    Book 14    Page 215
"Taphris is a spy," said Kenneth. "She has been sent to the stables by the Mistress to spy on you."
"I see," I said.
"Beware of her," he said.
"I will," I said.
Fighting Slave of Gor    Book 14    Page 250
"This morning," said Hci, "Watonka, and you two, left the camp of the Isanna."
"He was spying on us," said Iwoso.
Blood Brothers of Gor    Book 18    Page 84
"Are there spies in the city?" I asked.
"Doubtless Argentum has spies in the city," he said.
"Our spies," I said. "Ones who spy on our own people."
"Of course," he said. "That is a realistic precaution in any city.
"And to whom do these spies report?" I asked.
"To the proper authorities," he said.
"I am not aware of receiving the reports of these spies," I said.
"You are still being trained in the governance of Corcyrus," he said.
Kajira of Gor    Book 19    Page 176
"The rearward contingents of the units before us will be some ten pasangs up the road," he said.
"How many troops are there, altogether?" I asked.
"A great many," he said. "Are you a spy?"
"No," I said.
Mercenaries of Gor    Book 21    Page 36
"How do you call yourself?" he asked.
"Tarl," I said.
"You are from Port Kar?" he said.
"I have a holding there," I said.
"Are you a spy for Ar?" he asked.
"No," I said.
"Perhaps for Cos?" he asked.
"No," I said. I put the knife on the desk, before him. "Your sympathies, I assume, are with Ar," he said.
Mercenaries of Gor    Book 21    Page 147
"You would be mad to enter this place," said another voice.
"He is a spy, who would see behind our walls, who would inquire into our defenses," said another.
"No!" I said. "Blindfold me, if you will. Take me to Aemilianus!"
"You have been seen," said another fellow, the voice drifting down to me. I saw his hand, pointing out toward the Cosian lines.
I turned about. I could see one or two fellows standing at the height of the trench.
"Your friends call to you," said a voice. "Make it back to them, if you can."
I saw the crossbow move. Then, in another crenel, I saw another.
"Do not fire!" I called.
"Spy!" called one of the fellows.
"No!" I said.
Renegades of Gor    Book 23    Page 180
"You are a spy," I said.
"So, too, then must you be," she cried, "that you have been put in with me!"
"It seems they think so," I said, irritably.
Renegades of Gor    Book 23    Page 197
"There is something going on to the north, there," said a man to me, as I passed a guardpost.
"Where?" I asked.
"There," he said.
I could see the light of torches, could hear, distantly, shouts of men.
"I think you are right," I said.
"What is it?" he asked a fellow approaching.
"They are looking for a spy," he said.
"Do they know what he looks like?" I asked.
"They say he is a big fellow, with red hair," said the man.
"I have red hair," I said.
"If I were you, then," said the man next to me, "I think I would remain inconspicuous for a time."
"That is probably a good idea," I said.
"It would be too bad to be mistaken for the spy," said a fellow, "and be riddled with bolts or chopped to pieces."
Vagabonds of Gor    Book 24    Page 53
I suddenly knelt back. "You!" I said. "You are the spy! You are with them!"
"Yes," she laughed. "I am with them!"
"It is for that reason you wished to interrogate me," I said, "to see what I might know, or have guessed."
Vagabonds of Gor    Book 24    Page 111
"You admit that you are a Cosian spy?" I said.
"Yes," she said, hesitantly.
"Truly?" I asked.
"Yes," she said.
"Speak loudly and clearly," I said.
"I am a Cosian spy," she said.
"More clearly, more loudly," I said.
"I am a Cosian spy," she said.
"Excellent," I said.
Vagabonds of Gor    Book 24    Page 193
I thought, surely the fellow was right, that the circle, or its vicinity, was no place for a free female. It was a place, rather, for the joy of masters and their slaves. Similarly, the vicinity of such places, though I did not think it would be so in this camp, at this particular time, can be dangerous for free women. For example, sometimes free women attempt, sometimes even disguising themselves, to spy on the doings of masters and slaves.
Magicians of Gor    Book 25    Page 50
"He is a spy," said a fellow. "Kill him."
Torgus drew his weapon.
"We do not know he is a spy," said the fellow who might once have worn the scarlet.
"He is a spy," said Torgus.
"If so," said the fellow, "better to hold him, to bind him, and keep him for questioning."
Swordsmen of Gor    Book 29    Page 135
Outside the pavilion I had turned to Tajima. "You are to spy on me?" I had asked.
"I fear so," he had said, "Tarl Cabot, tarnsman."
Swordsmen of Gor    Book 29    Page 251
"It must take great courage to spy here, in such a camp," I said.
"I was well paid," he said.
"I think you are very brave," I said.
"I wagered, I lost," he said.
Swordsmen of Gor    Book 29    Page 384
"At least one is a spy," said Lord Nishida.
"Which?" I asked.
"I do not know," said Lord Nishida. "What do you think I should do?"
"I do not know," I said.
"I could kill them all," he said.
"Some would do that," I said.
"Would you?"
"I do not think so," I said. "I would probably dismiss them, send them away, on some pretext or other."
"Might that not arouse their suspicion?" he asked.
"Perhaps not, if it were subtly done," I said, "perhaps mixing them with others, but it would doubtless prompt the spy or spies to act."
"Or the assassin to strike?"
"Yes," I said.
"I will proceed differently, with patience," said Lord Nishida. "A detected spy may be of value. A spy regarded as undiscovered is not a spy to be replaced. Too, it is a spy who may be used to convey misinformation, lies, deceits, false plans, and such, to an enemy."
Swordsmen of Gor    Book 29    Page 424
Our attention, I fear, had been on our own business, pursuing a small group, some four or five, of Panther Girls. These were suspected of spying on Shipcamp, doubtless to report to some larger body.
Smugglers of Gor    Book 32    Page 321
"Genserich," said my master, "speculated as to the possibility of two large and complex forces, each of which might well have spies in the camp of the other, perhaps even highly placed spies."
"I recall," he said.
"I think you are such a spy," said my master.
"Possibly," he said.
"For whom do you work?" asked my master.
"I do not know," he said.
"You are hired through agents," said my master.
"Of course," he said.
"To what end?" asked my master.
"To inquire into the doings of Tyrtaios, and others," he said, "to see if deceit is practiced, to see if there is treachery amongst the Pani, to see if the cards are marked, the dice weighted."
Smugglers of Gor    Book 32    Page 529
"Who?" I demanded.
"Sumomo," he said.
"Then," I said, "she is the spy, or in league with the spy, or spies."
"Yes," said Tajima.
Rebels of Gor    Book 33    Page 111
I had hoped, of course, that I might find some way to relate to Adraste, in such a way as to better facilitate the charge that had been imposed on me. If I was supposed to spy on her, she, her environs and interactions, as seemed the case, the pretense of friendship seemed a plausible stratagem.
Quarry of Gor    Book 35    Page 179