Start with installing
gimp package in your Libertine Container.
Screenshot is GIMP running in Libertine Container using the Color 48 theme to make icons usable on a mobile device - https://bitbucket.org/paynekj/paynekj-gimp-scripts/raw/2e8e87faf5eaaf6036e0d8b68ff9f6f37a3f0421/themes/Color-48.7z
In order to install this theme I had to extract the 7z archive on my computer and upload the folder to the phone. Then I had to go into
home/phablet/cache/libertine-container/xenial/rootfs/usr/share/gimp/2.0/themes rename the "Default" theme folder as "Old Default" - and then paste in the Color 48 theme folder renamed as "Default" (because trying to change the theme from Gimp's gui would just crash it)
Also to get the GTK menus to be sized to be useable on phone it is important to: create a file named .Xdefaults in a text editor (tedit is a good Ubuntu Touch native one for this) with a single line Xft.dpi: 280 - or whatever other dpi value which you prefer. Save this to the home directory of your chroot environment - usually
.local/share/libertine-container/user-data/xenial/ in your normal home directory.
You also still need to use an external bluetooth keyboard for GIMP on the phone in order to have the gui respond.