You have to buy the Deadfire Pack DLC first (free) on GOG ; and it will show in your library.
https://www.gog.com/game/pillars_of_eternity_deadfire_pack
I tried redeeming the DLC through GOG, but it says "Some of the selected products require you to own the base game." Should I ask GOG for help, or the devs?
Does anybody know how to download the Deadfire Pack if I bought the game from Amazon? The files seem to be GOG, but even if I import the folder to GOG Galaxy, I'm not allowed to download the pack without having the game bought directly from GOG.