For older releases such as bionic, (xenial to focal) recent updated to Gimp 2.10.30 from Rob Savoury
I replaced the Gimp 2.10.22 appimage in kubuntu 18.04 a couple of days ago with 2.10.30 It does pull in several re-built packages that might affect other applications. So far no problems.
As previous post the Panda Jim PPA will get 2.10.30 in focal and later. I am using that in a kubuntu 20.04
Problem with these is no python 2 support, unless you install the packages yourself. Not difficult, just an annoyance.
My two-euros worth. For most Gimp editing the Gimp 2.10.22 or 25 appimage (with plugins) is more than capable, and if you need the gimp gmic plugin, well…there are up-to-date versions available.
There was a similar request over on gimpchat.com a couple of weeks ago. I pulled out some old tools and made a 2.10.30 appimage in a 'buntu 21.10 VM Lots of it hand assembled and it works there. It looks like it also works in Debian ‘bullseye’ While I included the Gimp python packages, not able to get these 100% working in Debian (at least not so far)
In a Debian VM looks like this.
If you want an updated gmic plugin that works with the older QT5 found in the appimage then I keep some here: gmic_gimp_qt and Ubuntu 18.04 Download, unzip, put in the appimage profile ~/.config/GIMP-AppImage/2.10/plug-ins/
That should supersede the old out-of-date plugin. If it does not work, well just delete it to go back to the old plugin.