To summarise for other folk passing by:
Intention
Backup photos on Phone onto (Linux) PC with a usb-c cable.
Problem
In a linux graphical file manager ,with the phone auto-mounted by gvfs, then a naive copy of files from the GrapheneOS pixel will not preserve the original photo's date / time attributes.
Documentation
Best starting point might be here: https://wiki.archlinux.org/title/Media_Transfer_Protocol
Solutions
gmtp : if this works for you, then gmtp gives you a nice graphical interface right-click-camera/download-files and will offer not to replace existing files already downloaded.
So, this gives you a one way sync for free, which is very nice and makes this the 'easiest' GUI solution.
There are options to keep the date/time attributes: settings/Preserve file timestamps: Enable this option to have downloaded files timestamps modified to match those as on the device.
jmtpfs : this integrates nicely with a graphical file manager using /etc/fstab.
go-mtpfs : setup with command line from terminal each time, but then you will get a nice mount point device in your graphical file manager or use cp -a to preserve file attributes.
eg. mount pixel/profile with: $ go-mtpfs pixel-profile & and unmount with $fusermount -u /home/user/pixel-mount-points/pixel-profile