Do you know if one can still use Google Authenticator w/ grapheneOS?
Discussion
No idea
Yes, everything and all apps work on GrapheneOS if you decide to install the services and framework. GrapheneOS is not rooted/jailbroken.

But it doesn't have to be Google Authenticator, use a FOSS alternative like Aegis from F-Droid.
+1 for Aegis. Way better than google Authenticator. You can directly import all codes from authenticator to Aegis.
Google enabling cloud and internet support for authenticator now is sketchy imo
It should be possibile, but why don't use the FOSS Authenticator Pro?