If I remember well, the GP template for TouchOSC used numbered names like knob1, knob2… button1, button2… So you you will have to give this kind of OSC handle name and activate OSC for the widgets you want to control from GP. If you monitor the OSC messages sent by TouchOSC you will see the address that you will have to use with GP.
Here are a few links that you could find useful:
There is also a nice introduction video to OSC networking if you need it.