Since this thread has a high Google rating for this issue, I’ll post the solution that worked for me. Ubuntu 19.10’s gimp-python can be manually downloaded and installed into Ubuntu 20.04. One other completely missing missing package, python-gtk2, also needs downloading. All other dependencies can be installed normally from 20.04. No source fidelity problems since it’s all from Ubuntu. No forced installation to ignore missing dependencies. The python-dependent “Heal Selection” works, and unlike the flatpak version, GMIC QT works.
wget http://archive.ubuntu.com/ubuntu/pool/universe/p/pygtk/python-gtk2_2.24.0-6_amd64.deb
wget http://archive.ubuntu.com/ubuntu/pool/universe/g/gimp/gimp-python_2.10.8-2_amd64.deb
sudo apt install gimp gimp-plugin-registry gimp-gmic
sudo apt install python python-cairo python-gobject-2
sudo dpkg -i python-gtk2_2.24.0-6_amd64.deb
sudo dpkg -i gimp-python_2.10.8-2_amd64.deb