Dunkelwelpling: I found a fix. Just go to "Connect Platforms", connect with Xbox Live and then click on "disconnect". Your games you bought on Xbox won't show up either, but i think most of us only subscribed there instead of buying something.
This fix worked for me.
I think the lingered "owned" games in Game Pass remained because the Microsoft authentication/authorization expired in Gog Galaxy. After reconnecting, I clicked on sync integrations, then back in the settings I disconnected Xbox, then quit Gog Galaxy completely, and upon restart I didn't "own" game pass games anymore.