muntdefems: As I said, all Feral (XCOM, Shadow of Mordor, Empire: TW, Company of Heroes 2) and Aspyr (Bioshock Infinite, Borderlands 2, Geometry Wars 3, Civ5, KoTOR II) ports fall into this category. And they only get paid by the original dev for the Mac/Linux sales of the games. So they could (and presumably would) have their Mac and Linux versions here if GOG had any means of telling apart a Windows sale from a Mac/Linux one.
With the exception of KotOR II, none of the aforementioned games are here at all, so even if Feral and Aspyr were here, that still wouldn't mean jack without the owner of the games in question agreeing to bring these games here. Let's also not forget the times that Linux versions aren't on GOG due to technical reasons (Metro: Last Light Redux and Victor Vran). As for KoTOR II, Aspyr did comment that they are in talks with GOG to bring their Mac/Linux ports here.