I had the same symptoms. Try the changes to the code I made in pull request 95. It uses a newer Linux API to register the device, and libinput seems happy with it now, so it works in gimp and inkscape.
Does GfxTablet give you a Touchscreen or a Tablet?
I’m trying to get Krita working with GfxTablet. Presently it doesn’t on my system, because the input driver gets recognized as a touchscreen instead of a tablet. I’m interested to know if this is distro level weirdness - I am preparing a patch to the driver to fix it, but I want to know if it’s going to mess with non-Debian distros.
So, can any users quickly check
most /var/log/Xorg.0.logafter they start
networktablet, and tell me if the thing is being detected as a touchscreen or a tablet?