Exsurgent: E.Y.E. - Divine Cybermancy (2738): Missing both LAN and online multiplayer features, missing French language. Note: The game was released as "Single-Player Edition", so technically it's a different game from the Steam version, but still...
vv221: It would be worth double-checking, but I think the "Single-Player Edition" is ~20% cheaper than the full version sold through Steam.
In my opinion (this is of course debatable) this makes it a lesser offender than most of the other games in this list.
True, but I figured I'd just list everything that's not blatantly out of place and leave it up to the editors to decide which of these they put in the official list. You'll find my post also includes some cases like the Fahrenheit: Indigo Prophecy Remaster not being released here, which I, personally, wouldn't put on the list because it's technically a different game (isn't it?)
EDIT:
Exsurgent: Wasteland 3 (2651): Missing "Colorado Survival Gear" DLC on GOG
ChrisG_: I can say this isn't true as this DLC is downloadable in my offline installer, perhaps some didn't wait a few hours on launch day before reporting it.
I should have worded that better. To clarify: The DLC
exists on GOG, but it can't be purchased separately. It's only available as part of the Colorado Collection, so if you've bought the game before that collection got released, you can't get the DLC unless you pay for the entire collection again.
Exsurgent: MENTIONS WHICH I DON'T FULLY UNDERSTAND (SORRY):
Battle Chess (2724, 2728, 2729, 2735, 2736)
Grargar: Battle Chess on Steam has two versions (floppy disk, CD-ROM)
Battle Chess on GOG only has the floppy disk version.
Thanks for clarifying, re-worded the post.