Thought I'd share an issue I ran into tonight in case it affects someone else. I bought the game tonight on GOG.com, and when I went to install, it opened up the Windows Store, which prompted me to buy GamePass or purchase the game.

After tinkering around, I found that I had synced my Xbox account with GOG, and that when I had done a trial of GamePass earlier this year GOG decided that Children of Morta was to be played through Xbox, even though I now owned it on GOG.

Disconnecting my xbox account from GOG Galaxy immediately resolved the issue, and I was able to Install and play the game.