Seen a lot of issues with the Galaxy application on the forums. I just did installs the ole straight d/l and install way off GoG web site.
One thing you have to make sure of when installing the DLCs and Expansions is that you have the latest version of the game. I wrestled for hours with one install when I downloaded the last Expansion Pack on the release date after downloading and installing the latest updated version and DLCs, etc on the day before. But it was actually only 2 hours before ... say 10 pm did all the updating of what I had and then at 12:10 am d/l'd the expansion pack.
The problem was the dropped a updated version of the base game at midnite too. And, tho I don't remember the actual numbers, but it was like say 1.10.12 and 1.10.21. Glancing at it .... with just the last 2 digits reversed I didn't notice that I didn't have the latest version.
So ....
1. Suggest not using the Galaxy app
2. If this is a first install done with a freshly downloaded game, make sure it is the latest version by searching for any patches and installing them before installing the latest DLC package