I installed the Deus Ex Revision mod and I can't seem to get it work. The original GOTY version works fine but if I try lauching Revision from GOG Galaxy it shows a critical error. I thought reinstalling it and the mod seperately would work but it hasn't. I'm not the most tech savy so any help would appreciated.

Maybe this thread could be of help?

Fix, to get Deus Ex: Revision Mod working on Original Disc and GOG GotY without Steam

Edit: Otherwise, you might want to try the Offline installer instead of Galaxy. Cannot say anything about it and possible required changes attached to it.