mtruostuff Hi there, I hope you don't mind but I have edited your post to replace the term ROM for OS. The use of ROM in this regard for GrapheneOS is a misnomer. GrapheneOS is a fully production ready OS and not to be confused with cherry picking hobbyist style projects from which the incorrect term ROM is derived (CustomROM).
As far as your actual question goes, GrapheneOS ships with our own Secure Camera making use of the CameraX API. This means that it can make use of all the features available when vendor extensions are available. Google hasn't released them for the Pixels yet however on a shot for shot basis using the standard mode our Secure Camera goes toe to toe for picture quality trading blows with GCam with access to all lenses.
However if you wish as you alluded to, you can indeed use GCam on GrapheneOS which then functions exactly the same as it would on a stock Pixel. If you don't wish to install the full sandboxed play services suite of apps you can simply install GSF (Google Services Framework) and GCam will function fine with all the processing and lens availability as you are used to.