Yes, you can. In order to do that:
1. Make sure Thronebreaker is updated.
2. Make sure that you’re online.
3. Log in to your GOG.com account that you want the goodies on, the option is available through a Multiplayer tile in Thronebreaker.
If you already completed your Thronebreaker play-through before getting GWENT on Steam, please log in to GOG.com via the Multiplayer tile in Thronebreaker, load a save where you have progressed the most.