Off the top of my head there is no reference to a toilet. I don't even remember a reference to running water. However, these are a couple instances which come to mind to answer your question.
In the center of the street is a gutter. It seldom rains in Tor, but the gutter serves to collect waste, which is often thrown into it, through open doors, by slaves.
Tribesmen of Gor    Book 10    Page 38
At the foot of the stairs, as is common in insulae, there was a great wastes pot, into which the smaller wastes pots of the many tiny apartments in the building are emptied. These large pots are then carried off in wagons to the carnaria, where their contents are emptied. This work is usually done by male slaves under the supervision of a free man. When the wastes pot is picked up, a clean one is left in its place. The emptied pot is later cleaned and used again, returned to one insula or another. There is sewerage in Ar, and sewers, but on the whole these service the more affluent areas of the city. The insulae are, on the whole, tenements.
"This is a sty," said Marcus.
"Do not insult the caste of peasants," I said. "It is the ox on which the Home Stone rests." Thurnock, one of my best friends, was of that caste.
Not everyone is as careful as they might be in hitting the great pot. Lazier folks, or perhaps folks interested in testing their skill, sometimes try to do it from a higher landing. According to the ordinances the pots are supposed to be kept covered, but this ordinance is too often honored in the breach. Children sometimes use the stairs to relieve themselves. This is occasionally done, I gather, as a game, the winner being decided by the greatest number of stairs soiled.
Magicians of Gor    Book 25    Page 272
