I'm on the same boat:  I just purchased the DLC in GoG, and got access to a patch.  It downloads as an installer, but it doesn't seem to "patch" the original, it creates an icon in my GoG games folder with a ghostbuster [circle-bar] symbol on it.

Is there some special incantation I must recite?  Or perhaps I have to swear loudly for it to work first?  ... cuzz I'm about to...

  -dZ.