Google Play Services on GrapheneOS are just regular apps you install. The compatibility layer that GrapheneOS adds just makes it so they can learn to live with being installed as regular apps.
I feel like there might be a misunderstanding here, so I'm just reiterating that there's no special sandbox for the Google apps. They're sandboxed the exact same way as with other apps you'd install.
Our Features page and Usage guide contains a lot more information about how it works which you might find helpful:
https://grapheneos.org/usage#sandboxed-google-play
https://grapheneos.org/features#sandboxed-google-play