I have to presume somebody is buying these books, or else why would they still be published? If he's still milking it after all these years, the teat has not run dry.
We have no way of knowing how much they sell of course. Since we are talking about what is now a self-published series, we do not need to assume that they are selling. If you don't hire editors, artists and so on, the cost of self-publishing is so low that you can do it even if you don't sell anything. You only lose the time you spend writing, but for many people that's a hobby. I mean, go to any fanfiction site, plenty of people are writing novel-length stories for free, just because they enjoy it.
One advantage this series has is name recognition. This stuff sold in its time, and a number of people remember it. I have read that the worst enemy of a self-publishing author is obscurity. No one knows you or your book, so no one buys it. The Gor series still has people who remember it, some of them even fondly.
So I would be surprised if this is not selling several hundred of copies per book, but I would also be surprised if it's selling much more than that.
Looking back at this thread, it's surprising that it has a lot of one-post contributors. People who apparently subscribed to the forum only to post their opinion in this thread. When that happens, you usually assume that those are hardcore fans of the series who have found out about this thread and are rushing here to gush about it. However, most of them seem to be rather balanced comments, admitting the rather serious flaws this series has but expressing some fondness for it.
I remember that in my early teens in Spain I found the Spanish edition of these books in the bargain bin after Christmas (that was the time when bargain books were usually liquidated). As a SF&F fan with little purchasing power, that was a highlight of the year for me, and I often found books by authors I loved that way, really cheap. I think about 12 of these Gor books had been published in Spain. So when I saw these books, extremely cheap, I saw that they were heroic fantasy with an erotic component, and I bought several of them. I suppose most users here are old enough to remember when there was no internet but, if anyone younger is here, I can't emphasize it enough: there was no internet. That means that the combination of softcore eroticism with heroic fiction was more attractive for a reader of that age than it would be now. And they had some virtues. I mean, reading JunkMonkey's funny summary of one of them, it still sounds to me like something that could be fun in a trashy way. It's a pity Norman hadn't been a better writer. Not a good writer, mind you, because good writing would be wasted here, but just not so actively bad.
I do wonder, though, how the endless war between the Priest-Kings and the Kurii ended up. Seriously, the worldbuilding and overarching plot was not so bad, as far as thouse sword and planet series went. It's the writing and, particularly, the dialogues that were painful.