I have the same problem. I've set up vmware with windows and traced packets which windows driver sends to device. Try the following program
http: / / dimak.fopf.mipt.ru/stuff/init-gpen.tar.gz - it just sends one setup packet to device but after it device switches to 'tablet' mode
For correct switching to absolute mode you should
1. unload usbhid driver (rmmod usbhid or if it built in your kernel you have to compile new one with usbhid compiled as module
2. run ./init-gpen
3. load usbhid driver
You you will try to do this, please, reply, how it will work.