Ok - here is how it works. Keep in mind this is really more of configuring the response curve of a particular parameter on a particular patch, not a "global" response curve for the pedal.
On the GM, select the patch where you want change the pedals response curve
Press the MOD button
Use the outer ring of the encoder knob on the right side of the GM, scroll to select the MOD# / CC# you have defined
Now push the center of the right encoder knob, you should see the 1st item in the "controllable parameters". On my GM, this is "PHA Speed" and it is set to "Off".
Use the outer ring of the right encoder knob to find the parameter on which you wish to change the response curve
After you've scrolled to the parameter, press the center of the right encoder knob
You will see: MAP:MinOut X%
The X will probably be 0. This is where to set the bottom of the sweep. Use the center of the right encoder knob to select the value you want.
Now, use outer ring to move to the next item (by turning it to the right). That should show: MAP:MidOut
This sets the middle of the sweep
Use the same process to go to MAP:MaxOut which controls the high value of the sweep
Lets say, for example, that you are using your pedal to control volume (which makes it more simple as volume goes from 0-100%).
Setting MinOut to 0%, MidOut to 50% and MaxOut to 100% would be a normal, linear curve
Setting MinOut to 0%, MidOut to 75% and MaxOut to 100% would mean you control 75% of the volume from the pedal in full-off to the middle of the sweep, and from the mid to full-on only 25%
For another example (a real one from my GM), I am using a CC pedal to bring a pitch shifter into my mix. I use the following:
Setting MinOut to 0%, MidOut to 25% and MaxOut to 45%
This is a mostly linear curve, but allows me to cap the pitch shifter at a max of 45% of the total allowable.
After you make your changes, remember to save your patch!
Since this is per parameter, you can use the same pedal to control multiple parameters at the same time, and each can have its own response curve... Even backwards (set to 100/50/0 rather than 0/50/100)!
Hope this helps