Could not read the contents of Desktop: Operation not permitted ~ GIMP ~ Mac ~ MacBook ~ macOS Catalina

I encountered the same issue as well. Allowing "Full Disk Access" for GIMP did not work for me (System Preferences > Security & Privacy).

What works for me is that GIMP 2.10.12 has access to the logged in user's "Pictures" folder on OS X Catalina so I just moved my photos to edit there. It's ok as a workaround.

It seems to me you only need to update the GIMP version https://gitlab.gnome.org/GNOME/gimp/issues/3710#note_659997 and that should suffice