I've seen a few other people have the same issue, but not a solution of any sort.

I bought the season pass, downloaded and installed the 1.3 patch and Flashpoint,
but Flashpoint content does not appear in game. Contacting GOG support gave me
the boilerplate "try it through Galaxy" response, which (as I do not use Galaxy) is no
help at all.

Has anyone had any luck fixing this issue?

I have the same problem and requested a refund, because i am unable to play the purchased DLC.

I just bought season pass as well...to my surprise the 3 dlc files were only 6mb each and are installers....which is useless for me. I hope gog can contact harebrained and get a offline installer made.

The DLC Files are only small, as the art and such is already in the base game.files Just run them and they are added ... you can see them when starting the game in main screen.