Not related to GrapheneOS.
Google Camera only accepts Google Photos as a gallery app

Photos required

To view your photos and media, install Google Photos from Play Store.

Is there a way or a patch for GCam to use another gallery app?

Yes. If using "gcam services provider", make sure to use the "photos" version, which is not available on F-Droid, only through github releases.

If using google binaries to satisfy dependencies, I believe there is a shim available, possibly from calyxos? Somebody more familiar with it should provide you with better details.

6 months later
a year later

Fried4650 I keep getting asked what app to open the photo with even if I click on always. Does this happen to you?