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.
GfxTablet not available in your language?
Thanks for reading this posting. If you’re interested in helping us with the GfxTablet translations, please visit GfxTablet on Transifex. You can help translating into your language online there, all you need is a free account.
If you don’t want to create an account on Transifex, you can of course take the original string files, translate them and send/attach the translated XML files.