Compared to other applications, games are relatively easy to port because a majority of the games assets are things like textures, models, sound, etc., that can be run on any platform, all other things being equal.
As to how difficult it would be to port a game from Windows to Linux, the worst case would require a partial rewrite of the game engine to use the appropriate Linux libraries (or whatever Linux uses these days) instead of DirectX. Compared to ports between PCs and consoles (or even just between consoles) where the game content itself often needs to be modified to work within the new platform's resource and input constraints, engine modifications aren't really that difficult.