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.
Possibility of passing mouse events through?
I’ve found that GfxTablet also works great for using a Bluetooth mouse on a computer without Bluetooth. Mouse movement is passed through as I would expect. Scrolling doesn’t work and middle click/right click all register as left click. Would it be possible to have GfxTablet also pass those actions to the computer?