Greetings,
I had the same problem. Went to this GOG support page and clicked the refresh link it provided (or you can search GOG support if you don't trust the link):
https://support.gog.com/hc/en-us/articles/360004019934-Unable-to-install-or-download-Pillars-of-Eternity-2-Deadfire-
Fixed it for me.