Since I still largely associate GOG with classic games its always wild to me when I see an older game that's on other digital stores but not on GOG. Of course there are games that aren't classic that would still benefit from being on GOG, mainly where being DRM-Free is a particularly large boon.

Personall my pics would be the Hexen/Heretic series and for a more modern game I think it would be nice to have the Jackbox games not tied to Steam, though I'm not entirely sure if that would work since they do connect to a server for most of the games.

Dusk seems an obvious choice, and is already DRM free on steam. Hexen and Heretic can be used DRM free from steam, just copy the wad files from them and run them through GZDOOM ir equivalent.

Grimoire - Heralds of the Winged Exemplar

Turmoil...I can't get enough of it atm.

Knights of Honor.

Extreme G-2, it's a racing game released in 1999 by Acclaim Entertainment. One of the first games I ever played on my computer, still have the CD. Unfortunately Throwback Entertainment has the rights now. Nightdive Studios that acquired some games by Acclaim, like Turok, did release their games here.

Should I say it? Yeah,okay...Skyrim.

Z.A.R. (including the elusive mission pack which was only released in Russia that not even Steam has).

I'd say Dark Messiah Of Might And Magic.

... and of course Dishonored 1 & 2.

Civilization 1,2 & 5

I'd say, in general, those with at least a few thousand votes that are on Steam -

https://www.gog.com/wishlist/games#order=votes_total - or well-received AAA games in general.

Hexen, Heretic, Skyrim, Final Fantasy #, Mass Effect 1, Max Payne 1 and 2, Age of Empires II HD Edition, and so on.

City of Brass. Excellent indie game with a lot of replayability.

All the good ones.

https://store.steampowered.com/app/469820/Genital_Jousting/