GOG Galaxy 2.0 is showing SPELUNKY from my steam library as installed when it isn't. Does anyone else have any issues like this?

You'll be able to permanently import all the games listed below to your GOG.com library – assuming you own them. ... You can connect only one Steam account with your GOG.com library.

Thanks, but that doesn't answer my question. Is GOG Galaxy 2.0 showing games as installed when they aren't on anyone else's library?

At least for me all games seem to be detected correctly. I don't have Spelunky though. Best I can think of is to report the issue to support.

Also, out of curiosity: What happens if you try to launch the supposedly "installed" game?

GOG brings me to Steam and Steam asks me if I want to download the game.

Install and uninstall(If a game is big you can simply cancel the download). It will remove the game from the list.
(Weird, I'm pretty sure I posted this solution earlier today in this very thread o_O)

Was Spelunky installed/uninstalled by someone that uses your games library through Family Sharing? I think that might be the cause. I noticed that I have Bad Company 2 listed as installed even though it was my brother playing it on his Steam account via Family Sharing. I never even touched it.

Thanks for the tip makwikus2, but it seems that the problem has been fixed. It's no longer labeled as installed and I personally haven't opened up the game before. My steam account isn't shared either. It's fixed so I guess that's all that matters. lol!

Steam keeps track of installed games in the Windows registry or file on macOS.
If you had game installed and did not uninstall it via Steam, but just removed it from the disk - Steam will not recognize the game as installed, but won't change the registry.

I guess Galaxy does read that registry to determine which games are installed.

There are so many threads about this issue, but none seem to explain how to fix it...

I know this is an old thread but I think I found a workaround for this problem. I was having this issue with Ac Valhalla. I had to go to settings and turn the integration with Epic Games and Uplay off (I bought the game through Epic but it requires uplay to validate it, apparently).

After I turned the integraton off and synced the games I turned it on again and the game was shown as uninstalled as it should be.

I hope it helps

UPDATE: It didn't work, it started showing within the installed games again

This not only fixed it for that game, but for all other games with the same issue, instantly. Thank you!

I got here a few years after this was posted but I found a solution for me for Steam games that were showing installed when they weren't:

Completely close Galaxy and delete:

steam_[..a bunch of numbers..]-storage.db file from %programdata%\GOG.com\Galaxy\storage\plugins\

(For me it was: C:\ProgramData\GOG.com\Galaxy\storage\plugins)

and

steam_[..a bunch of numbers..] folder from %localappdata%\GOG.com\Galaxy\plugins\installed\

(For me it was: C:\Users\[USERNAME]\AppData\Local\GOG.com\Galaxy\plugins\installed)

After the next Galaxy launch, you will have to reconnect to Steam but when I did this it properly showed the correct games. This might work for other plugins too though I haven't tested.

Hope this helps!