POLE7645: Works fine with me.
Requiem is installed automatically when you install Black Plague (probably because it's an expansion). It can even be launched from GOG Galaxy once it's installed. It's pretty obvious why you can't install it separately.
resevil239: okay, this definitely wasnt the case at all, least not for me. If anyone else comes across this in the future and has the same issue: skip using Galaxy and just manually download and install requiem form the site, it should then work fine.
After finishing black plague there was no way to start up requiem through gog galaxy and I couldn't find any indication that it was ever auto-installed. I'd previously just assumed it would dl it after the first start up of black plague, if it hadn't already. Despite being an expansion, it functions and installs like a separate game.
Maybe it's because I installed it with Desktop shortcuts, but GOG Galaxy made shortcuts for both, and both works.
Guess you CAN install it through GOG Galaxy, just not launch it. You need the desktop shortcut to launch it since you can't launch it from Galaxy (unless you directly use the executable).