I think i had 360 games on my GoG acc and today i login and i have only 289, after checking what games are missing, all of them are from Steam such as, (i can find them using ctrl + F but they will be on my ''others'' category instead of Steam and i don't want that):

- Hitman 2016
- Hitman 2
- Max Payne 3
- Assassin's Creed 2 (Uplay/Steam)
- Assassin's Creed 3 (Uplay/Steam)
- Assassin's Creed IV(Uplay/Steam)
- Assassin's Creed: Synficate (Uplay/Steam)
- Assassin's Creed: Unity (Uplay/Steam)
- Batman Arkham Knight
- DmC: Devil May Cry
- Devil May Cry 4...

...Ayways a lot of games!!

I'm tired of this, every time some games disappear, it's been almost 2 months since Dragon Age Awakening and the F.E.A.R dlc disappeared and i got only a generic reply from GoG and now all of these games... I can't use this launcher if things like this keep happening!!

Same thing happened to me. All my Steam games don't show up in Galaxy. But I don't blame Galaxy. The Steam plugin is the only one that constantly gives me problems. I have connected with my Epic, Origin, Xbox, and Uplay and those don't give me anywhere near as much trouble as the Steam plugin. So it may be on Steam's side that is at fault.

I noticed that the general number of owned games decreased today by around 40. Looks like some of the Steam games are missing indeed. Probably the Steam plugin messed something up. I hope it gets fixed soon.

I've this same problems. Almost 70 games from my list disappeared. Waiting for fix.

Yeah my steam games were only visible partly. after a reconnect it was not better. the second reconnect now is no longer able to connect to steam anymore...

Just want to report that all my Steam games are showing up in Galaxy now!

Today it is up again. it appears that the second connect was faulting. a restart of GOG Galaxy showed that stema was not connected. after the last reconnect it went back to operational.

Yeah for me the Steam games are back too, i also got the info that the Steam plugin is community made that's why some errors like that can happen often, fortunately the guy releases fixes very fast... The only official GoG integration are GoG and Xbox.

Also 4 games of my Steam are missing:
- F.E.A.R: Extraction Point
- Dragon Age: Awakening
- LEGO NinjaGO The Movie
- Mafia 3 Definitive Edition

These are the games missing for me

- Tumblestone
- Showdown Bandit
- Psichodelya
- Polyball
- Pinball FX3
- Mafia 3 Definitive Edition
- Lego Ninjago
- The Deed

I have also have games missing since I first started using Galaxy 2.0, those games are

- aMAZE Valentine
- Just Cause 2 Multiplayer Mod
- Skull Girls Endless Beta

Same here as well. I just compared what games are missing from my Steam library on GOG and 16 games aren't there. Another thing happened is that three games from Steam (TF2, Alien Swarm and Conqueror's Blade) aren't counted among my Steam games. I installed all three of em and it still didn't add them. But yea this is just infuriating at this point. As much as I'm loving GOG Galaxy, it keeps making troubles that is really annoying.

Update 2020-06-11
All games reported by me now appear in Gog Galaxy 2.0 (apart from F.E.A.R.: Extraction Point).

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Original post:

(All AppIds are IDs from Steam)

More missing
I found that the following games (which I own on Steam) are not listed in my owned games in GOG Galaxy 2.0:
- Burnout Paradise: The Ultimate Box (AppId 24740) - not sold on Steam any longer
- Mafia II (Classic) (AppId 50130)
- Mafia II: Definitive Edition (AppId 1030830)
- SEGA Mega Drive & Genesis Classics (AppId 34270) - currently not available in my country on Steam

Confirmed missing
I can confirm the following Steam games (which were reported by others) are missing for me too:
- F.E.A.R.: Extraction Point (AppId 21090) - it's not even present in the general search which has "all" games
- Mafia III: Definitive Edition (AppId 360430)
- Tumblestone (AppId 269710) [actually it's there, but only listed as owned on HumbleBundle)
- Showdown Bandit (AppId 1076280)

Just hard to find:
Perhaps the following game should be easier to find:
- OKAMI HD / 大神 絶景版 (AppId 587620) - in GG2.0 it's called "Ōkami HD" which is indeed the right title, but I think "Ō" should be findable by typing "O" in the filter - but: findable in the general search by typing "Okami"
- Through Abandoned: The Underground City (AppId 389270) - in GG2.0 it's called just "Abandoned"

Works for me
On the other hand, the following Steam games (which were reported as missing by others) actually DO appear in my list (as Steam games):
- Hitman 2016 (AppId 236870)
- Hitman 2 (AppId 863550)
- Max Payne 3 (AppId 204100)
- Assassin's Creed 3 (AppId 208480)
- Batman Arkham Knight (AppId 208650)
- DmC: Devil May Cry (AppId 220440)
- The Deed (AppId 420740)

No games were hidden by me in the client.