You can probably read the first three books without too much shame (Boris Vallejo covers aside), but you deserve all the scorn you'll get if you stick with the series, because after this, Norman really began descending into BDSM fantasyland.
In the third book of the original trilogy, we get introduced to the bug-like alien "Priest Kings," making Gor really more of a science fiction series, though its feel is much more swords & sorcery. I maintain that the first three books are a pretty good read if you like slashbuckling fantasy adventure, though it's certainly not epic literature. After this, though, Norman became increasingly obsessed with his sexual kinks, until long, tedious passages about female submission took up more of the books than the plot. (I skimmed a few of the later novels -- it's not even good