Timboli: The free Twinsen DLCs are pushing the concept of DLC somewhat, as they are standalone games, not additions or extras for the base games.
Yes, it is weird, but technically speaking they are added as DLCs in the GOG shelf.
Then again, soundtracks have been DLCs for a long time, and they don't add anything to games either, they just offer some content related to the game.
Personally I value original versions more, so I would prefer to have remasters, remakes, rereleases and such as DLCs to the original games.
But I guess they have their reasons to do things the way they do.
Anyway, missing from the list: Syberia 3: The Complete Journey.
Syberia 3: The Complete Journey includes Syberia 3 and the An Automaton With a Plan DLC.
And a bit controversial case: Syberia: The World Before Deluxe Edition.
The deluxe edition includes five goodies that the base game doesn't have, but the word "DLC" is not used anywhere. There is a product called Syberia: The World Before Deluxe Edition Upgrade, which on Steam is listed as a single DLC product, that upgrade is also available on GOG, but not listed as DLC. And obviously it upgrades the base game with the extra goodies that the deluxe version has.
So is it a DLC or not, and if it is, is it a single DLC, or do the number of goodies count, it's debatable.
But such a thing exists.