"Recently used" files are listed in a
~/local/share/recently-used.xbel file (which is normally shared by all apps).
If you use the Gimp flatpak version, this version has its own copy of
~/.var/app/org.gimp.GIMP/data) so has its own copy of the
~/local/share/recently-used.xbel file, so has its own list of recently used files.
You can't replace
~/.var/app/org.gimp.GIMP/data/recently-used.xbel by a link to
~/local/share/recently-used.xbel because this file is erased/recreated by Gimp (and the re-creation doesn't re-create the link). But you can replace
~/.var/app/org.gimp.GIMP/data/ by a soft link to
~/local/share/ and Gimp will happily share the file with others.
I don't know is this applies to other installation modes (snap, for instance) but this is fairly easy to try.