Well i have a ps2 controller and i have the same problem. there are official drivers only for xbox guitar as far as i know and maybe they work with ur usb ? ps3 guitar
I use a ps2 to usb adaptor and works fine with Frets on fire but not with GH.
So i tried a game profiler (joy2key that is free) and mapped the guitar buttons as keyboard keys. Everything is working fine but the star power is assigned to a button cause the sensor is not working