To start with, connect the pedal directly to the GTR. Don't use the Expressionator until everything is working.

Then, "Clear internal programs and setup." (See UM for details).

Make sure your pedal is connected to the right jack – it sounds like you may be using the "Foot Switch" input, rather than the "Foot Pedal."

For information on remoting parameters, look in the UM under "Parameter area" for "Midi Control" (page 35 in the release 7 version on the Web Site). Controlling a parameter by MIDI is essentially the same as using a pedal. Also look at "External Controls"  on the following page.