So basically, my dumbass wanted to uninstall and reinstall GOG Galaxy without know that there was the uninstaller in the package. I manually uninstalled most of files and the installer still detects some of the files grasping for life. Could someone please provide me with file locations so I can make sure there are non I missed?

It's going to be registry entries. You deleted the files, but the PC still thinks it's installed. The uninstaller would have removed them, but if you deleted them manually, you are going to have to remove the entry from the registry manually.

Can't help with pinpointing where they are exactly at the moment, because I'm not at my PC :/

Use an uninstaller to scan for leftovers, like
https://geekuninstaller.com
https://github.com/Klocman/Bulk-Crap-Uninstaller.

I would rather reinstall Galaxy again over the old installation, and then uninstall it. That should make sure you get rid of everything.

C:\Program Files (x86)\GOG Galaxy
C:\ProgramData\GOG.com\Galaxy
%LocalAppData%\GOG.com\Galaxy

Well, they can't really do that if the Galaxy installer recognizes Galaxy as already installed because of the leftover registry entries...

Look for a folder named "GOG Galaxy" or "GOG.com" within these directories. The default installation location for many applications is in the Program Files folder on 64-bit systems and Program Files (x86) on 32-bit systems.

I don't know about the Galaxy installers, but so far I haven't seen a Windows application installer that would have refused to install because of an existing (broken) installation.

I've used that trick several times when a program has refused to uninstall for some reason, or was removed only partially, including with a couple of GOG games.

Oh well, it doesn't hurt to try. If it refuses to install over an existing installation, then I guess it can't be used.