Found something that instantly made me revert from the #Flatpak version of #gthumb to the Fedora-provided RPM: opening an image file via Nautilus, gthumb is unable to see the other media files present in the same folder, so you can't flip thru or do batch operations. I recall the same limitation being present in Amberol.
I didn't see a ticket about this sort of pitfall in Flatpak's issues tracker. Should it be filed there, or is it already possible and an issue ought to be filed on apps?