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.
Small Feature Request
This is essentially the same request as last July - but I may have formulated it the wrong way. I’d really need to see the moves of the S Pen on the canvas of GfxTablet, for the sake of positioning the S Pen relatively to its previous positions, without having to look at the computer screen. Probably just a matter of putting a pixel at each (x,y) event, and an eraser on the top row tools to erase the canvas from time to time. This stuff can of course be made optional. My guess is that the extra code needed would be fairly small … (Slide Graphic Tablet does this, but has many other drawbacks, and is not open source …)