You can also get the appimage on https://www.gimp.org/downloads/

After downloading, set the execute bit: chmod +x ./GIMP-3.0.0-x86_64.AppImage and then open the file to open gimp.

If you dont mind using the terminal, there is topgrade which can update many different kinds of packages with a single command (topgrade).

You can also build mintinstall (linux mints updater/store) on ubuntu.

Did you use the linux-surface kernel? It has additional community maintained patches for surface devices and detailed installation instructions for the best linux experience. From their feature matrix they seem to have full support for sgo2.

Not sure if its available on pmOS though.