Since this thread is the top hit for a Google search for the gimp-python problem, I’ll post the solution that I found. Ubuntu 18.10’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 security problems since it’s all from Ubuntu. No forced installation to ignore missing dependencies. “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