I recently required Google maps because sadly none of the open source maps apps were cutting it to navigate to a specific place, so GMaps is sometimes required for me. Here's me setup in case it helps.
In a new user profile, I installed GPlay Store & Services
I allowed network to GPlay services but denied Play Store network. Then I fully disabled Play Store.
Now using Aurora Store, I downloaded Google Maps and allowed ONLY Network. Then I opened GMaps, did NOT use any Google Account, and I selected the area that I want to navigate in and I downloaded it for offline use.
I then disabled network access to GMaps and allowed Location. Now I can navigate using GMaps offline without an account.
To update the offline maps, I disallow location, allow network, update the maps, then deny network and allow location again.
Hope this helps you if you want/need a more private way to use GMaps on GrapheneOS!