I did get the manual offline installer to work, hopefully this helps others here.

If i could get GOG to let me submit that there is a problem they might get it fixed... but I currently haven't figured out how to jump through all the hoops to report this to them.

I have just reported the issue where Norse Lands DLC is not showing in GOG Galaxy after purchase. I am hoping that if you are having the same issue, when it is fixed for me it will be fixed for all of us.

Update: The DLC (Norse Lands) has now appeared in Galaxy on my install. Hopefully it is now present on yours too.