Yes, there are tattoos on Gor. Mostly in the jungles of Schendi and also in the desert region.
Here are all the places the word tattoo (and other variations) appear throughout the series.
There is also Body Paint.
I looked at the man who sat, cross-legged, behind the table. He was a large, tall man. He had long, thin hands, with delicate fingers. His face seemed refined, but his eyes were hard, and piercing. I did not think he was of the warriors but I had little doubt he was familiar with the uses of steel. I had seldom seen a face which, at once, suggested such sensitivity, but, at the same time, reflected such intelligence and uncompromising will. Following the lines of his cheekbones there was a stitching of tribal tattooing.
Explorers of Gor    Book 13    Page 151
It made clear that he, the Ubar, Bila Huruma himself, was one of them, himself an askari. His face had been broad, and the eyes widely spaced. On his cheeks and across the bridge of his nose there had been a swirling stitching of tattoo marks, the record of his transition, long years ago, into manhood.
Explorers of Gor    Book 13    Page 236
He was a large man, with tattooing.
Explorers of Gor    Book 13    Page 225
In the lips of some of the men were brass plugs. Facial tattooing, in various designs, was common. The opulence and color of the court of Bila Huruma was quite impressive. I was sure that it would have shamed the display and pageantry of many Ubars in the north. There were various racial types represented in the court, almost all black. I was the only white present. There were some brown fellows from Bazi, though, and one of the attending physicians was oriental. Even among very similar black types there was variety in hairstyle and tattooing, and dress, which I took as evidence of cultural or tribal difference.
Explorers of Gor    Book 13    Page 228
His face, like that of many in the interior, was tattooed. His tattooing, and that of Kisu, were quite similar. One can recognize tribes, of course, and, often, villages and districts by those tattoo patterns.
Explorers of Gor    Book 13    Page 253
"Such things are not of interest to me," said Shaba. The tribal stitching of tattoo marks on his dark face wrinkled with a smile. "But I do not expect you to believe that," he said.
Explorers of Gor    Book 13    Page 431
The girl, bound, knelt between the guards. There were tears in her eyes. Her head had been shaved, completely. She had no notion what had been written there. Illiterate girls are chosen for such messages. Originally her head had been shaved, and the message tattooed into the scalp. Then, over months, her hair had been permitted to regrow. None but the girl would know she carried such a message, and she would not know what it might be.
Tribesmen of Gor   Book 10    Page 23
The captain looked at me. "Sleeve," he said. I thrust back the sleeve of my shirt, revealing my left forearm. It did not bear the blue scimitar, tattooed on the forearm of a Kavar boy at puberty.
Tribesmen of Gor    Book 10    Page 83