Interesting stuff regarding playing Diablo IV via Gamepass. You DO need to use the Xbox application to load the game.
If you reboot your PC and open Battle.net application it will say you can’t play the game without any indication as to why, the Play button will be dim and be uninteractive, with the program seemingly indicating the game needs purchasing (lists prices of each edition on the right side). When you open the Xbox App (while Battle.net is still open) the Battle.net program will re-launch and verify Gamepass permission and then show the game as available to play within Battle.net
So it seems authorization to play the game persists for either 24 hours since Xbox App was closed or for the duration of the current Windows session (likely the former). ideally the expiry date would simply be programmed in to your Battle.net account instead of needing to open Xbox App every 24 hours to authorize you.
Edit: Hmm, maybe I’m wrong and it was just a bizarre intermittent issue? Or someone changed something on the backend without informing me.