cogadh: As long as GOG's licensing allows them to do that, which is the one thing that everyone seems to gloss over (even TET).
That is a really interesting question. How is a Linux license given? To "Linux as a whole" to different distros? Does GOG hold general licenses (including Mac)?
As GOG probably just acquired the distribution rights on the original licenses, which under which the game were originally released, it is pretty much open for legal interpretation if GOG can simply sell a linux version or if they need additional approval. (I'm pretty sure eg. there never was a Syndicate release for linux ... ).
Effectively, if using emulators, are you really selling a linux version? Or are you just selling the original Dos version made runable under Linux? Is that covered by the license agreement?
(And yes, that is the stuff I do for a living, although not with licenses and videogames sadly)