Some apps require google play, like android auto and maps.
I'm using grapheneos with open street maps, so I don't think there would be any problem with that. There is a work around to load some apps that require google play with out a google account. I'm guessing that is what would be affected.

Graphen is based on AOSP - Android Open Source Project. As the name implies, that's the part of Android that's Open Source. Google publishes this code, Graphene OS people modify it and release. All bugfixes, security fixes and new features go there. Google first stopped accepting contributions from other devs and now they are limiting how often will they release AOSP and how often will they publish security fixes. This makes development of Graphene OS more difficult and it looks like Google's ultimate goal is to make Android closed source. Graphene OS doesn't have the resources to develop Android by themselves so this would probably kill the project.