I just purchased the Rocktron Hex Volume/Expression Pedal. Using your suggested setup, I have been unable to calibrate the pedal using a stereo cable plugged into the expression OUT of the pedal into the expression Exp IN of the G-System. There is a real quick "taper" going from Min to Max from heel to toe on the pedal. It is basically acting like an on off switch.
The internal wiring of the Rocktron Hex is in reverse of what the G System needs to see in regards to using a TRS cable in the expression output of the Hex.
Basically a simple switch of the wiper and outside pin of the potentiometer wires or just switching places with the "tip" and "ring" wires inside the TRS cable will do the trick. You can also use the regular volume OUT of the pedal with a standard TRS cable.