[deleted] I'm now regretting not keeping better notes because I can't unpick other stuff I did subsequent to getting gmtp to work. What I can say is that I got gmtp working but then went off looking for a way to automate a two way sync, gos-pixel < > workstation so I could delete photos on the PC and sync the delete back to the phone and, as @[deleted] pointed out at the beginning of this thread, preserve file date/time attributes.
I must have used jmtpfs to get me most of the way because, just now, I found I had placed a device in Thunar named pixel-jmtpfs which mounts the GrapheneOS pixel in Thunar and allowed me to use Thunar to copy all the screenshots to a new folder on the PC and preserve date/time attributes. I had added entries in my /etc/fstab that set this up for me and I don't want to encourage people reading this to mess with their fstab; so let's just stick with getting gmtp working for now.
I do seem to recall now that one problem I had encountered with jmtpfs was that all profiles, on both pixels, came up with the same device identifier:
s@t490s:~$ jmtpfs -l
Device 0 (VID=18d1 and PID=4ee1) is a Google Inc Nexus/Pixel (MTP).
Available devices (busLocation, devNum, productId, vendorId, product, vendor):
4, 23, 0x4ee1, 0x18d1, Nexus/Pixel (MTP), Google Inc
This was too much of a hazard for me and risked accidentally syncing over the wrong device/profile (folks at home, this is why a SYNC is NOT a BACKUP). At that point I went back to gmtp as the easier temporary fix to backup photos ... and I'd not been back there since until your messsage caught my eye. I think I'm going to hide away from this again for another few months :)