Mori_Yuki: games with sexual or pornographic content, which I don't believe GOG has ever or will ever sell such
If you mean "live action sexual or pornographic content", then no, I don't think they will pursue it either. Otherwise Subserve is pretty much it in terms of animated sexual or pornographic content, or better said a game centered around it. If you have any doubts, feel free to browse the developer's website and check out their other... materials :P.
amok: a bit too late... gOg is already selling games wirh 'sexual or pornographic content' such as Hunipop 1 and 2, and hey sold games like Lula: The Sexy Empire
Those are indeed sexual, but not pornographic. No sure about Lula, since I've never played it, but in Huniepop there's only innuendo beside some (optionally) graphical artwork you unlock.
Subverse and Succubus are pushing into the pornographic territory, which was not previously a thing on GOG (outside of maybe some patched graphic novels - I can't say because I'm not a fan of those).