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.
Hello @d3vid, you might like to try the changes I made in pull request 95, which use the newer Linux kernel ioctls for configuring uinput devices. I made the changes in response to the “missing tablet capabilities: resolution” error, just as you saw in your list-devices output. After the changes, my setup recognises the tablet and I see it in gimp and inkscape.
The firewall solution would certainly work on a trusted LAN, thanks for the suggestion. As a proof of concept GfxTablet definitely succeeds in demonstrating that we have are carrying functional drawing tablets around with us every day. Thanks!