Dragon's Dogma is still outdated.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.