I was wrong:
When I had Turboprint running OK, that was on GIMP 2.10. It appears it does not support GIMP 3 at all. What is more, it seems there are no plans to make it support GIMP 3, but they (Turboprint) are looking at creating a standalone app for loading and printing images.
And as far as I can see, and as confirmed above, neither does gutenprint support GIMP 3.
Back to GIMP 2, I guess. Which is very annoying, as I'd written some python GIMP 3 plugins, and they are NOT the same as GIMP 2 ones.
When I had Turboprint running OK, that was on GIMP 2.10. It appears it does not support GIMP 3 at all. What is more, it seems there are no plans to make it support GIMP 3, but they (Turboprint) are looking at creating a standalone app for loading and printing images.
And as far as I can see, and as confirmed above, neither does gutenprint support GIMP 3.
Back to GIMP 2, I guess. Which is very annoying, as I'd written some python GIMP 3 plugins, and they are NOT the same as GIMP 2 ones.