I believe that IMEI is not accessible to Sandboxed GPS because GPS is treated as a user app.
To quote chillcat from the GrapheneOS matrix room (sorry chillcat if this is inappropriate, note I'm not speaking for chillcat by quoting them) when re sponding to: "Is there any identifier the sandboxes play services can see which would allow this sort of cross-profile correlation (like IMEI on a normal installation)?"
mainly battery level, full list of apps in a profile, storage left, sim country code and ip address can theoretically be used to correlate 2 user profiles
From this and my own understanding, I assume that means IMEI and MAC address are not available to Sandboxed GPS. Someone please correct me if I'm wrong.