GIMP printing problem on Ubuntu

It sounds like you have the "Snap" version of GIMP installed...

When you search for GIMP in Ubuntu Software, you should see two results if you're using Ubuntu 18.04 LTS ("Bionic Beaver") or newer:

If you cannot see two versions of GIMP, you will need to enable "Community-maintained free and open-source software (universe)" by searching for "Software & updates" in the menu (its icon looks like a cardboard box with a globe/Earth), then make sure there is a "tick" in the box that says "Community-maintained free and open-source software (universe)":

Snap packages often have problems with "seeing" hardware - at least in my experience - so I'm guessing you've probably installed the Snap version of GIMP... Install the "traditional" version of GIMP (i.e. the other result) instead.

The "Snap" version will have "Snap Store" listed under "Source":

Whilst the "traditional" version of GIMP will have "ubuntu-bionic-universe" listed under "Source":

Just don't forget to remove/uninstall the Snap version of GIMP first - otherwise you'll have two copies of the same program installed and two icons in the menu! You can do this by selecting the "Snap" version of GIMP in Ubuntu Software.

Before removing snap's gimp, you can try this suggestion.
TL;DR: run

$ sudo snap refresh gimp --edge
$ sudo snap connect gimp:cups-control

Worked for me.