I don't have a list specifically for GOG, but I do have a general list. I don't really give a shit if these land on GOG, or Steam, but I'd buy these 10 games again given the chance. So with that said, games on this list will probably not be sold anywhere, instead of just wanting DRM free versions.
1. Soldier of Fortune 1 & 2 (Rights hell.)
2. No One Lives Forever 1 & 2 (Rights hell.)
3. Halo Master Chief Collection (Console exclusive, but with the release of Halo Wars Definitive on Steam, things are looking up a bit for this coming to PC perhaps?)
4. Wolfenstein '09 (Used to be on Steam, got taken down, I assume because it switched hands from Activision to Bethesda?)
5. Prey '06 (Used to be on Steam, got taken down because they ran out of keys, never came back. With the new Prey
coming out, it would be cool to see a re-release of the older one.)
6. Resident Evil 3 (Since it will be a while before we ever get a REmake of 3, I'd kill to play the original again.)
7. Onimusha (The first Onimusha is a fantastic RE style game. Needs to be brought back!)
8. Metal Gear Solid 1-4 (Good MGS ports to PC would sell like hot cakes.)
9. Silent Hill 1-4 (Ditto.)
10. Powerslave/Exhumed (The same person behind the engine of the recent Turok remasters and the D64 port were also doing a port for this, but unfortunately someone got mad and ceased his development.)
Honorable Mention: Command & Conquer series (I know this is on Origin and I own it there, but I'd prefer it on GOG or Steam.)
RayRay13000: -Half Life 1-2 along with Episodes 1-2 -Portal 1-2
Hell will freeze over and Satan will step down from his throne before that ever happens.