I usually used offline installers and installed the game like that.
But recently i started using gog galaxy mostly for faster download speeds and i noticed valhalla hills downloaded a 701 or 710 mb patch?
I did have the two horned addition and installed the dlc in offline installers but really don't know what the patch was for in galaxy.

Edit: i guess if the dlc is already installed with offline installer gog galaxy installs it again as dlc get added extra even though you already have them installed