I had the same problem that started today. I don't know how to fix it but you can downgrade an older version of both git-gui and gitk which seems to fix this for me

# show available versions:
apt-cache showpkg gitk

# remove installed version
sudo apt-get remove gitk

# install one version from the apt-cache. For me it was:
sudo apt-get install gitk=1:2.34.1-1ubuntu1

# do the same for git-gui:
sudo apt-get install git-gui=1:2.34.1-1ubuntu1