I do not see the 1.3 update in the offline installer download section of my library. I have purchased the season pass, and I do see the 5mb download for Brushes with Death. Turns out Brushes with Death requires the 1.3 patch that seems to be missing from the offline installer download section. I do not use Galaxy, as the exclusive reason I buy on GOG is the DRM-free offline installer option for each game. Surely GOG is not behind on releasing the required update for DLC while taking money for that DLC. What am I doing wrong here?

Nothing. It just takes a while until the update packages for the offline installers are created. This is an automated process and unfortunately takes a while.

But there will probably be a patch soon anyway, as there are some minor and major bugs. And you have to wait again ;)