Run the appimage from a terminal and see what is reported.
Two instances, and there might be more with your installation.
There are missing resources, this for gmic_qt
/home/rich/.config/GIMP-AppImage/2.9/plug-ins/gmic_gimp_qt: ./lib/x86_64-linux-gnu/libgomp.so.1: version `GOMP_4.0' not found (required by /home/rich/.config/GIMP-AppImage/2.9/plug-ins/gmic_gimp_qt)
There are duplicate plugins and the one in your Gimp profile is ignored.
Skipping duplicate plug-in: '/tmp/.mount_V5z6Rq/usr/lib/gimp/2.0/plug-ins/sharpen'
It is also a matter of philosophy. Should the appimage come pre-loaded with scripts and plugins that might or might not be required or should it be a 'vanilla' version. There are many existing compiled plugins that do work with an appimage, and of course anything.py should work providing it is not too old.
I go for the latter and my homespun Gimp 2.9.5 appimage based on the 'buntu ppa packages will load gmic_gimp_qt, albeit the 2.8 version from gmic.eu
Unfortunately trying and failing to build gmic_gimp_gtk/qt for the last week against the 'buntu 2.9 packages. Starting to hate cmake, qt ...etc.