It's all in the title. As far as I know, the developers also made a Linux version for both games, so why are they not being sold on GOG?

GOG does not currently support Linux so no Linux versions until they officially announce it. Considering MAC support was added only few months ago, I would not expect Linux support for at least another year - year and half. And before you ask, no, GOG does NOT offer unsupported binaries/packages. There are many requests (probably hundreds over the years) asking GOG to provide unaltered games in zip or iso format but they've always (so far) been turned down due to support requirement (someone WOULD complain to support if the unsupported files would not work on for them even if GOG clearly stated they would not offer support for them). But considering increasing number of games offering Linux versions, I'm sure GOG will add support eventually.

Ok, I see. Thanks!

I'd only ask if it might be possible to turn the demo of the linux version into the full game providing the game files from GOG. As in I do a Windows install using WINE, install the demo and then copy the files over like you do it with DOOM 3 or Quake 4. If THAT would be possible, GOG wouldnt have to support anything.... I should try to investigate that someday :(