The gas pedal of a steering controller is probably more like a potentiometer than a switch. But, the principle is similar because both are implemented as three-terminal devices. The switch terminals are Common, Normally Open (NO), and Normally Closed (NC). The potentiometer terminals are End 1, End 2, and Wiper. What a potentiometer does is create a variable voltage divider, where the wiper acts as the midpoint between two resistors to the ends. The series combination of the resistors always stays the same: so in a 10K pot, it is always 10 kohms; in a 100K pot, it is always 100 kohms, etc. The relative balance of the two resistors changes as you move the pot. In this way, if the two ends are connected to Vcc and ground, the voltage at the wiper will move between Vcc and ground as the balance of the resistors changes (voltage divider).
An important idea here is that the potentiometer is symmetric with respect to the two ends. When the wiper is closest to End 1, its voltage is equal to the voltage at End 1. When the wiper is closest to End 2, its voltage is equal to the voltage at End 2. So if your pedal outputs 0V when fully depressed, the wiper is closest to the End connected to ground at that position. To reverse the polarity of the wiper output, all you need to do is reverse the wires connected to End 1 and End 2. That's it.