Dragon's Dogma is still outdated.
Missing Languages: Missing Traditional Chinese.
Missing Features: There's no photo mode for GOG. The online features don't work.
Missing Updates: Missing updates since June 2017.

Children of Morta seems to be missing Family Fireside Fables, a free audio book DLC.

I just sent an e-mail to their support. Hopefully it gets answered.
(also, GOG could use an easy way to link devs/publishers' support's contact info like Steam does)

About Forager, the link for the source doesn't work anymore (now it leads to the forum's main page), but the source is still accessible through Internet Archive:
https://web.archive.org/web/20201021144621/https://steamcommunity.com/app/751780/eventcomments/2951502108439423944/

Also, according to this comment and the ones afterwards, the game was update a few times here after that. But it seems that it's missing the 4.19 update now.

The entire Metro series. Linux ports for Steam but not for GOG.

When was the last time A Hat in Time got updated on GOG?

January 24, 2020. To version Nyakuza Metro 1.12.

Since then Steam had at least two updates:
- February 11, 2021.
- May 4, 2021.

That includes Metro Exodus which had a Linux version released on Steam on the 14th of April, five weeks ago now. I have very little hope left.

It's said in the list that Hyper Light Drifter is missing a hotfix update, but according to GOGDB, at least the Windows build received the hotfix.
Not sure about the Linux version, as it doesn't seem to be tracked on GOGDB (just reported the issue in the GOGDB thread).
And I imagine the Mac version wasn't updated because it's a 32 bits program, and so it's incompatible with the newer builds of the MacOS.

The newly released game Biomutant includes only the mp3 soundtrack version. Steam buyer can get the WAV files too.

Thanks to Windforce for bringing it to my attention.

28 May 2021
60 Parsecs!
Mac support added and Windows updated
Currently on par with Steam (version "1.3.2 Build 26")

GOG needs to mandate a new policy that should force devs to update their GOG builds to be in line with the Steam version within two weeks.

Dragon Dogma - has not been updated since 2017. Neither the developers nor Gog.com does anything.

Fictorium- achievements not working and developer doesn't want to fix it

King of seas - has no achievements and the developer does not respond to messages from gog users

Not a Hero - it takes years to update

Freeman: Guerrilla Warfare - does not have steam updates. It also has no achievements. The developers blatantly lied.

Universim - Has no achievements in gog although the developers said they were going to add them.

Warhammer 40,000: Mechanicus- many updates missing

Star wars kotor -
It has no update, literally abandoned.

Gog.com has acted regrettably. The developers are a shame.

This is a shame and would be solved if cd project forced developers to update their games in the contract but cd project and gog.com just want our money.

And there are many more cases.

They already have such clause in their contracts afaik.

Fictorum received an update only on GOG very recently, do you happen to know if it fixes the achievements issue?