v3: You sure about Steam? Don't know what they're doing but take Journey of a Roach, The Last Tinker: City of Colors and The Book of Unwritten Tales as an example. Only there are their Linux versions present, not on HB (though they don't offer TBoUT DRM-free on any platform) nor here. They're either doing the porting themselves or prodding/working with original developers somehow. But I'm guessing it's more of an in-house job, since it would stand to reason that Linux versions would also be available elsewhere otherwise (in case the developers themselves have ported them).
I'm pretty sure that aside from providing the tools for SteamOS versions of the games, Steam has absolutely nothing to do with development/monetary assistance of Linux versions. Why are some Linux versions only on Steam? Reasons. For Age of Wonders 3, for example, Triumph mentioned that providing Mac/Linux versions of the game to GOG is not economically feasible for the time being.
v3: That's certainly second best to helping Wine support new games, which I agree is different story althogether, but one that would be nice if GOG became a part of. As far as tinkering goes, there's already a number of PlayOnLinux scripts that target specifically GOG versions of various games, so users have done a tremendous job of making it easier than ever to install Windows-only games in Linux. Well-known company like GOG should try and go a step further.
They could, but it's always a case of technical/legal issues. Will said PlayOnLinux scripts work on any Mint/Ubuntu system without any problem? They have to ensure that they will be, because GOG promises support/30 day money guarantee in case it doesn't work. As for legal issues, since GOG started supporting Linux less than a year ago, they would have to renegotiate their deals with every publisher to allow them to sell Linux versions. If the publisher doesn't want to renegotiate for one reason or another, then there is little that GOG can do about that.