Title says it all - if you use google apps that don't require network access (like dialer, camera, photos, clock, etc.) with the network permission disabled is it still possible for them to send harvested data to Google's servers?
Google Apps Private with Network Disabled?
It's possible to have a "leak" if you are using Play services and allow Network access since those apps will use it for additional functionality. However, most of those apps work fine without Play services or only require Google Services Framework which you can deny Network access for.
13 days later
For anyone looking at this question later, the grapheneos websites says:
GrapheneOS adds a Network permission toggle for disallowing both direct and indirect access to any of the available networks [...] the Network permission toggle prevents apps from using the network via APIs provided by the OS or other apps in the same profile as long as they're marked appropriately.
Which seems to pretty clearly answer the question.