I don't think you can assign the gain of the amp to the pedal.  But I do know that you can assign the gain from the stomp to the pedal.  One other thing you could do is set up two rigs.  One being clean with the volume is set so when the heel of the pedal is down your clean channel is at 100%.  Then you set the other channel as the distorted sound you like.  Then you set the pedal when the heel is down your distortion is at 0%.  So the effect when you roll from heel to toe on the pedal your clean goes 100%-0% and your distortion goes 0%-100%.

You have other options as well.