Hi. Just dusted off the old gt6 and would like to setup an expression pedal. Im using the gt6 on a raised shelf at waist level (easy selection of settings) but would like to have a foot pedal for the wha. I have a Bespeco VM 18L pedal and would like to use this. The instruction manual has not been fruitful so far. I know the pedal works as the tuner/pedal function works. Will this pedal work as an expression pedal for the wha or is it only a volume / on/off switch? Thanks for any help.
Looks like it should work. It has a polarity switch so you may have to play with that.
They list these in their knowledge base.

Roland EV-5
Roland FV-500H
Roland FV-300L

I bet the $30 M-Audio EX-P will work too. It's possible that you may need to cycle power on the GT-6 in between changing the polarity switch, if it detects the pedal on startup.