Yeah, this is now almost comically sad. I've been asking on the Pathea discord forever now, and if you believe Pathea, they apparently did submit the DLC content to Team17 with this update to be published to GOG. That obviously didn't make this update.
After this latest update went live, I asked them again about it and I got the reply:
Sarcosmic: We're still trying to make DLC for GOG/Epic to happen
But realistically, if Team17 can't even be bothered to update the Patch Notes correctly so they display in GOG Galaxy, what are the chances they'll be bothered to fix this? It's kindof sad...Consoles really did get DLC before GOG, and at this point I believe GOG may end up being the only platform to NOT get DLC.
If Team17 doesn't have the resources or capabilities to actually support a given platform they shouldn't publish there. Way back before the Aug 2019 update, they should have just shut it down and offered Steam Keys.