I have 2 titles I want to add as owned (as they don't want to get integrated from Steam), but can't find them on Galaxy, even though they do are present on IGDB. Wasn't that Galaxy's database?

https://www.igdb.com/games/estranged-act-i

https://www.igdb.com/games/penumbra-requiem

EDIT:

Here's 4 more I can't seem to find on Galaxy, but I do find on IGDB:

https://www.igdb.com/games/sakura-halloween
https://www.igdb.com/games/sakura-moonlight
https://www.igdb.com/games/sakura-sweetheart
https://www.igdb.com/games/command-and-conquer-tiberium-alliances

EDIT2:

Seems the Sakura games are now available in Galaxy, but the C&C, Penumbra and Estranged games still are not.

EDIT3:

Found another one that's on IGDB, but not on Galaxy.... https://www.igdb.com/games/afraid-of-monsters-directors-cut

EDIT4:

This: https://www.igdb.com/games/sonic-dash worked until today, got integrated from Xbox/Microsoft, but as of today not only did it disappear from my library, you can also no longer find it to manually add.

Very odd,
I was about to answer on your other topic that you can add missing game variations/versions to IGDB yourself, but apparently that isn't a live link to the database? Or a selective one.
Don't know what goes wrong there, but it's an interesting find.

Estranged: Act I and Afraid of Monsters are classified as 'mods' on IGDB.
Command & Conquer: Tiberium Alliances is a 'web browser' game.
It could be that GOG (for the time being) has excluded these types of entities in Galaxy.

The reason behind Penumbra: Requiem could be that it is weirdly configured on Steam.
It has a separate store page but it can not be bought on its own.

Silent Hill: Alchemilla, Half-Life: Uplink, Nightmare House 2 and Underhell are all mods I have manually added.
Penumbra Requiem is listed as a main game. And even though it does not get integrated from Steam, it should work to manually add it (Alien Swarm, Codename: Gordon, Minvera: Metastatis and Team Fortress 2 are all Steam games that have not been integrated directly, but managed to manually add), but it doesn't.

But about Command & Conquer: Tiberium Alliances maybe you are right, but seeing how we can not find a reason why the other are not shown in Galaxy you might also not be right.

EDIT: Alslo Sonic Dash worked until today, got integrated from Xbox/Microsoft, but as of today not only did it disappear from my library, you can also no longer find it to manually add.

OpenMW and Hexen: Deathkings of the Dark Citadel also have IGDB pages but don't appear in GOG Galaxy. I couldn't find anything in the pages themselves that indicate why GOG skips over them.

It is very annoying that some of the games are on Igdb, not listed on GOG Galaxy. I guess it may be related with the search function. It only shows up to 30 games. Maybe we could find the games we were looking for if it would allow us to see more than 30 games.