Add Age of Civilizations II. At the very leat a new map of Asia was added towards the end of December 2020 which is not in the GOG version. User Yanx also reported last year about a new map of Europe, that was added in december 2019, but is nowhere to be found in GOG version.

Shadowrun Returns, Swag and Sorcery and The Darkside Detective appear in the list but have no issues listed. I'm not sure whether they are wrongly listed there or the editor forgot to put what's wrong with them.

For The King is outdated (disabling crossplay multiplayer in many cases, due to version incompatibility) and missing the Lost Civilization Adventure Pack DLC (available at DRMed stores)

I believe Darkside Detective was on there due to missing soundtrack. Soundtrack is available now though and has been for a bit.

The issue with Shadowrun Returns was missing Deluxe DLC, but it has been fixed since.

"Swag and Sorcery" is missing (again) an update that is on steam since months if I'm not wrong : last update on steam is 1.54 ( https://steamcommunity.com/app/929010/eventcomments/2972901184175247451?snr=1_5_9_ ) while GOG's version is still at 1.53 according to gogdb ( https://www.gogdb.org/product/1781942763#changelog ).
That's why I still haven't bought the game at this sale btw...

I didn't see these two on the list:

Not a Hero has achievements on Steam but not GOG
Not a Hero: Global MegaLord Edition includes a Making Of documentary on Steam but not GOG

Unruly Heroes has the soundtrack as free DLC on Steam but not GOG

Terminator: Resistance's GOG version has had broken achievements for two months- you can see the list of them but they're impossible to get. Someone contacted the developer about it directly but has been completely ignored. Details here: https://www.gog.com/forum/terminator_resistance/achievements
Minor issue compared to actual missing content, but I find it quite annoying all the same.

Regarding the missing "Pipe Dreams" (which is free on Steam but not on GOG) by the devs of the Simulacra series, apparently they still are planning to bring it to GOG though it might still take some time.

"Hi there, yes its still in the works. But as I said, we are focused on wrapping up Dr.Who now and we are in the midst of SIM3 as well. So its gonna take some time before we look at bringing pipedreams to GOG."

Well, that explains a lot about my experience of the game so far. At this point I'm wondering if I should just quit halfway through and revisit it in a year, when it's finally been fixed... if they don't just abandon the GOG version. I prefer to pick GOG over Steam whenever possible, but crap like this doesn't make it easy.

Edit: Pointed out below that the content is not missing from Gog's version, so this is definitely not a valid entry.

Bethesda managed to combine 2nd class treatment and DRM-pushing in a single move.

According to this post they created a patch for The Evil Within ... and then locked it behind the requirement of tying a Steam account to a Bethesda account. Yay for piling DRM on top of another DRM.

Obviously it's unavailable for us who do not have steam accounts.

Apparently, someone at Teyon have been reaching out to GOG via their Support network for a VAT inquiry and it wasn't handled well - 2 months without a response. I'm stunned that partners don't have a direct line to someone at GOG for something like that. If Teyon has been handling communication this way, no wonder they have trouble with GOG.
https://twitter.com/PawelKlimesz/status/1356526562837938176

Further down that thread, you will see that GOG's version already includes this content, and is up-to-date.

There are
two
DLCs
missing from New Super Lucky's Tale on GOG.

Edit: Nevermind.

According to the post below, the DLCS are only for Super Lucky's Tale.

Those are only for Super Lucky's Tale, while GOG only has New Super Lucky's Tale. See the disclaimer on those pages: