Help understanding cross progression

While I'm at work waiting to go home to merge I've seen a few people mentioning that if I play through gamepass I won't get the benefits of the cross progression.

Just a basic background I'm a new for honor player with xbox ultimate gamepass which allows me to play on both PC and Xbox.

I was really hyped up to get my reps and heroes together. Will I be able to get it or not? Or do I have to purchase the game on both platforms to be able to merge?