Ok, here goes a list of all games I own on GOG that aren't shown on Galaxy 2.0 library. Let's add yours if you find something so that we can have a nice list and help have this issue fixed:

Age of Wonders Planetfall
(It has not yet been released, but neither has CP 2077 and it is correctly shown)
Disciples 2 - Dark Prophecy and Gallean's Return
(Disciples 2 - Rise of the Elves is shown)
Quest for Glory I
(All the rest Quest for Glory games from the package are shown)
Rad Rodgers - Legacy version
(The standard version of Rad Rodgers is shown)
Sword of the Stars: The Pit Osmium Edition
(The older Sword of the Stars: The Pit version is shown)
The Incredible Adventures of Van Helsing: Complete Pack
The Incredible Adventures of Van Helsing III
(part II and Final Cut are shown)
XCOM: Enemy Unknown Complete Pack

Can confirm

QfG1 and XCOM, added them to a bug tracker entry:
https://mantis2.gog.com/view.php?id=977

It seems the issue is fixed right now.

For me, it's Van Helsing 2 not being shown. Had to check on the store if it was still available. I realize this thread's a bit old on internet standards, but I still face the issue.

EDIT : Hm. For some reasons, it shows Van Helsing 2 on Steam, not on GoG.

To clarify I bought Van Helsing 1 at release on Steam and bought the Complete pack recently on GoG. Bit weird.

Maybe you should report it as a bug ?

I've had the issue with Syberia 3. Reported the bug, but it's not fixed just yet.

Can also confirm Congnition Episode 1 doesnt show but episode 2,3 and 4 are showing fine? very strange

I've had the same problem with Congnition Epsiode 1, raised it as a bug.

These games are not showing in my library in Galaxy 2.0:

Arma 2: Combined Operations
Ashes of the Singularity: Escalation
Haegemonia - The Solon Heritage
King Arthur II: The Role-Playing Wargame + Dead Legions
Spellcasting 1+2+3
Star Wars Jedi Knight: Dark Force II - Mysteries of the Sith
The Guild 2: Pirates of the European Seas

Spellcasting 1+2+3
Star Wars Jedi Knight: Dark Force II - Mysteries of the Sith

Same here. More or less those are the only GOG titles not detected in my client. two of around 350 is not a bad result :)

Mysteries of the Sith seems to be fixed now, It is shown in the library

Yesterday Mysteries of the Sith appeared in my library, but today seems to be hidden again.

greetings

These ones are now listed in Galaxy:

Haegemonia - The Solon Heritage
Spellcasting 1+2+3
The Guild 2: Pirates of the European Seas


These ones are still not listed:

Arma 2: Combined Operations
Ashes of the Singularity: Escalation
King Arthur II: The Role-Playing Wargame + Dead Legions
Star Wars Jedi Knight: Dark Force II - Mysteries of the Sith