Try looking at the existing posts first, and I'm posting this for other readers, more than the OP, has both topics have exactly the same title, and the correct answered in the first topic Will Requiem ever be installable through GOG Galaxy?
As is stated in that thread
It is installed when you install Black Plague (since it's an expansion rather than a stand-alone game). The shortcut is in the start menu.
Why this happened.
Basically when GOG originally packaged existing Collection/Bundles, as single installs.
Things like Penumbra Collection, and Tomb Raider 1 to 3, and 4 to 6, though separate games, were one install, and one entry in the Library, and it was a long standing mass user request to, unpack such Bundles, even if sold as a bundle.
When GOG eventually did this, it produced this anomaly with Black Plague's Requiem (Stand Alone) Expansion being listed as a fully separate game, instead of the DLC it actually is.
The offline installer, require's Black Plague be already installed, and never really creates a new game install, though as a stand alone expansion, it gets a separate Desktop Shortcut.
The discrepancy, is caused because GOG Galaxy, is still incorrectly auto installing all DLC/Expansions.
Instead of giving the user the optional choice.
I don't even use Galaxy, so I don't know whether it auto installs DLC/Expansions, if you own them, or not.
It should be as optional on Galaxy, as it is offline, but it doesn't appear that it is, and is yet another reason Galaxy fails to be the fully optional client it claims to be.