FrAnubis: It does show up for me after trying it out as part of the game pass. Keep in mind that, from my experience, you need to have launched the game at least once with your Xbox profile for it to show up on GOG.
xanasago: Stupid Question, but how do you launch the game with the xbox profile? I just launch the game with the xbox(beta) app or directly.
When launching it it signs you into your Xbox profile automatically (when it shows the "syncing data" window), using the account your Windows 10 profile is linked to (you can have a different account and profile in the Windows 10 Store independently of the one you use to play and use the rest of the PC).
So yeah, just launch it once and it will now appear as played in your Xbox profile and thus should appear on GOG Galaxy, you may need to resync integrations though.