Can confirm. I see the updated offline installers too. Can't believe it took almost a month.

Got in contact with Oriental Empires dev. He said that he will try to upload the update to GOG. Although he also said that it's problematic to upload to GOG and that currently his internet is not so great.
We know that the former is not really true, but since Oriental Empires generally had a good track record with updates I'm willing to give him the benfit of the doubt about the latter. Anyway we should see whether he'll update the game in the next few days.

It was almost half a year for some of the latest Star Trek games, the offline installers were all release candidates while the Galaxy version was final. A month is nothing :)

Of course they probably had identical content, but were named differently.

Yeah, this is unacceptable.

Inscryption: People already noted in December that GOG users do not have Kaycee's Mod (free DLC expansion). It's currently in Beta on Steam, so maybe we'll get it when it's fully released, but it's been out for 2 months and could end up being one of these "forever beta" things. Even if we do eventually get it, it's frustrating to miss out on all the community discussion around this effectively released game mode. Also, GOG version lacks cloud saves.

Myst 2021 edition: Lacks all of the achievements.

I just asked in Red Blue Games' Discord group: And then one of the devs replied this: (cut part of the message to avoid noise)

Hopefully it works out!

edit: added a new message to the quote from the dev.

After cross referencing gogdb and steamdb it seems that Wytchwood is outdated. Last update on GOG 14.12.2021. Last one on Steam 10.02.2022.

There's also some bureaucracy on GOG's part that delay update releases here. I'd give the devs 2 weeks before adding them the list.

The Incredible Adventures of Van Helsing: Final Cut has multiple offences:

1) On GOG it's sold as one 4-title pack while on Steam is available as 9 separate titles (The Incredible Adventures of Van Helsing: Final Cut+The Incredible Adventures of Van Helsing Anthology)
2) 2 DLCs are not available on GOG: Van Helsing: Veteran Multiplayer Skin and Van Helsing II: Magic Pack
3) Soundtrack on GOG is missing 2 songs (The Incredible Adventures of Van Helsing II - OST)

On the other hand, it's worth noting that the game is way cheaper on GOG. Final Cut alone is 45 EUR on Steam while all 4 games are 40 EUR on GOG.

Just downloaded the OST via Galaxy and can confirm that are not included in the zip file.

edit:
Another game I don't see on the list
NecroVisioN: Lost Company
missing
German language.

"This is the Presdient" hasn't received a patch released two months ago on Steam. https://steamcommunity.com/app/730430/discussions/0/3203744999893263419/#c3203746177244893111

Shenmue III Deluxe Edition is missing DLC available exclusively on Steam:
The Burning Sandstorm technique and Flight Jacket are NOT available with the GOG Deluxe edition but unlock for Steam and Epic Games Store Deluxe Editions.

Full details here: https://www.gog.com/forum/shenmue_iii/a_warning_regarding_achievements_dlc_and_saved_games

I haven't seen this one in the list:

Dead Age 2

The last gog version is 1.1.2 while Steam got three additional patches with bug fixes:

version 1.1.5 - 2 dec 2021
version 1.1.4 - 1 oct 2021
version 1.1.3 - 16 jul 2021

I do not know the specifics of the modifications except for 1.1.3 which had:

"We have fixed two situations in the GUI. Selling objects at the merchant and modifying of equipment had a few problems in the GUI plus the latest update with Controller improvements."

I have asked the dev by email if they are planning to bring the game up to date on Gog.

Midnight Protocol is missing Linux version, which exists on Steam.

GOG page says "Note: Linux version will be available soon" since "forever".