https://grapheneos.org/usage#android-auto
VoLTE and VoWiFi are supported if your carrier supports it.
Google Maps and Uber work fine, provided you install sandboxed Google Play.
Android Auto is completely disabled as an opinionated security measure.
Google Pay and some banking apps (ones that require Google's Play Integrity API) will not run. GrapheneOS doesn't attempt to spoof these APIs because they are moving towards cryptographic verification [0]. Most apps don't require this check but if they do you are out of luck unless you can get the app developers to trust GrapheneOS's keys.
[0]: https://grapheneos.org/articles/attestation-compatibility-gu...
Nowadays it's the Google's Find My Tag network which will be unsupported.
It's not a real security check that they're doing, but rather just checking for certification, which is very unfortunate.