that's what finally worked for me:
- uninstalling GOG Galaxy (for those with a slow internet connection: this will NOT affect your downloaded games, they'll stay right where they are)
- reinstalling GOG Galaxy from
https://www.gog.com/galaxy
- importing all games (they all got some auto-patch for whatever reason)
- verified Gwent
- going to my account -> settings and privacy -> order history
- all pending purchases suddenly popped up (they didn't before the reinstall)
- picked one and completed it
- verified Gwent AGAIN (because the kegs didn't show in-game after completing the purchase)
- all kegs there!!