DrIstvaan: Unfortunately, the very fact that Steam sells those games may imply we won't see them here. After all, why sell Doom/Doom II, for example, here, for $10 for everyone, when you can sell it for €10 for European users?
DelusionsBeta: That's what they said about Activision.
However, if you compare the relevant titles (there're not many of them, namely only the King's Quest series and the Space Quest series), you see that in those cases, GOG's version is actually more expensive, so it makes (financial) sense; probably Activision demanded they be broken down into packs of 2-3 games so they cost more.
Anyway, I didn't say it's a 100% sure sign those games won't come here, but we can't exclude the possibility.
And indeed, with well-worling, popular source ports, you only need certain files, and it doesn't matter wether you get them from GOG or Steam.
Still, I guess
many users would buy them here if they were available.