I cannot for the life of me work out how to use the expression pedal in the G3X as a simple volume pedal.

I have tried following what it says in the manual about setting parameters and assigning functions to the pedal for other effects, and I have no problem getting those to work, but when I try applying the same methodology to set the G3X for use as a volume pedal, it does not work.

Any help would be much appreciated

Thank you.