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.
PPA for GfxTablet available
I’ve just finished my first foray into Debian packaging, and hence a PPA is now available at http://launchpad.net/akdor1154/gfxtablet. You can add it to your Ubuntu system with
sudo apt-add-repository ppa:akdor1154/gfxtablet sudo apt-get update sudo apt-get install networktablet
It’s identical to what is in rfc2822’s master repository. Currently it needs to be run as root (or else grant your user access to
/dev/uinput) but I intend on fixing this.