Just why Norman's novel proved so successful is an interesting question, complicated by the fact that the Gor series enjoys a great deal of notoriety nowadays for themes implicit in its early novels that only come to the fore later on. Tarnsman of Gor could, I think, be reasonably described as a sword-and-planet tale in the Burroughsian mold, but shot through with a decidedly un-Burroughshian philosophy. It tells the story of an Englishman named Tarl Cabot, who works as a history teacher in New Hampshire before he is whisked away to a planet occupying the same orbit as our Earth but on the other side of the Sun. Norman tells this story in the first-person, a stylistic debt he clearly owes to Burroughs, even if, as I said, the rest of the novel isn't particularly Burroughsian in its themes or tone.
Upon his arrival on Gor, Cabot learns that the immortal Priest-Kings have taken men and women from Earth at various times throughout history and set them on this other world, for reasons that are inscrutable. For the most part, the Priest-Kings take no interest in the affairs of humans, allowing them to behave as they wish.
“There is at least one area, however,” ... “in which the Priest-Kings do take a most active interest in this world, and that is the area of technology. They limit, selectively, the technology available to us, the Men Below the Mountains. For example, incredibly enough, weapon technology is controlled to the point where the most powerful devices of war are the crossbow and lance. Further, there is no mechanised transportation or communication equipment or detection devices such as the radar and sonar equipment so much in evidence in the military establishments of your world.”In this way, Norman creates a world in which Earth men must fight as did their pre-modern ancestors, but without all the messy and difficult aspects of living in those times. It is, to be blunt, the kind of world many gamers like for their RPG settings, preserving the "cool" aspects of the past while jettisoning those things that might make it unpleasant. Perhaps it's for this reason that you'll find lots of references to Gor in fantasy and science fiction fandom during the late '60s and early '70s, including in the roleplaying hobby. Dave Arneson's Blackmoor, for example, incorporated some elements from Tarnsman of Gor and other early novels in the series, but the impression I get is that these elements were, like those from Tolkien and many other sources, very superficial and not at all indicative of having adopted the philosophy of Gor's author.
“On the other hand,” he said, “you will learn that in lighting, shelter, agricultural techniques, and medicine, for example, the Mortals, or Men Below the Mountains, are relatively advanced.”
Which brings us to the elephant in the room when it comes to Tarnsman of Gor -- its philosophy. I never read a single word of John Norman until I was an adult, but, when I was a kid, I distinctly remember there being hushed tones and knowing glances whenever the word "Gor" came up in gaming circles. This baffled me, all the moreso after I'd read issue #61 of Dragon which included a write-up of Tarl Cabot as part of the "Giants in the Earth" series. Writer Glenn Rahman (of Divine Right fame) judged Cabot's alignment as Lawful Evil, a decision that occasioned at least one letter to the magazine questioning his decision and much bafflement from me, as, back then, I naively thought that evil characters didn't make good protagonists. Why did Rahman think Tarl Cabot was evil and, if he was right, why were gamers so taken with this character?
When I finally did get around to reading the first few Gor novels, including Tarnsman, it soon became apparent. Gor might crudely be called a "Nietzschean" world where slavery is not only a common and "natural" practice but treated as beneficial for both slave and master. This aspect of Gor is not strongly evident in the early novels, though it's definitely there implicitly, and, given the large role in plays in later novels, it has come to be the aspect nearly everyone mentions when discussing these books. "Gor" has become a byword for deviant sex and misogyny and it was on this basis that it was treated as something "secret" among the older guys I knew involved in the hobby.
In Tarnsman of Gor, though, this stuff is very much in the background. Instead, we're mostly treated to the story of Cabot's transformation from a boring college professor to a boring swordsman and rider of giant birds. And that, for me, is the biggest bafflement I have about the Gor novels: they're pretty dull. Cabot isn't a very interesting character; he comes across mostly as a mouthpiece for Norman's views on politics and morality and is disappointingly cerebral. He's no John Carter, who, for all his Virginian stolidness, is at least compelling to read about. The same cannot be said of Cabot, who's about as boring a fantasy character as any, despite the occasionally intriguing world he inhabits.
I can only chalk up the popularity of the early Gor books like Tarnsman of Gor to the fact that, at the time they were published, there was a hunger for fantasy -- any fantasy -- and so mediocre volumes like these were eagerly snapped up. You might think that prurience played a large role in their success and that may be the case, but I do wonder about that. As I said, the first few books don't dwell on these aspects and those that do are, like Cabot, rather dull and uninteresting, being far more focused on philosophizing than titillation. For my money, Howard and Leiber provide a greater erotic charge than Norman, but then both those authors created memorable characters and stories whose primary purpose is to entertain rather than inculcate the reader in their own worldview.