The potentiometers on the gShield are setting the Vref (Voltage Reference) level on the TI stepper motor driver chips. This determines the current limit for the driver.
You can measure this voltage at one of the potentiometer pins (see photo) and adjust it to obtain a given current limit. Use great care as the potentiometer pins are very close to other components and it is easy to short the pins together with your meter probe. Really bad things happen.
The gShield (version 5) uses a 0.1 ohm current sense resistor so the formula is Vref = 0.8 * I.
The maximum current the driver chip can deliver with appropriate cooling is 2.5 amps. To use this current value you would set the Vref voltage at (0.8 * 2.5 amps) = 2 volts. This is the theoretical current limit.
The NEMA 17 motor (part #25253-01) that Inventables sells has a maximum current rating of 1.68 amps per phase. To set the maximum current limit for this motor you would set the potentiometer to (0.8 * 1.68 amps) = 1.34 volts.
It should be noted that the maximum current a motor can handle is not necessarily the correct setting for the motor. Stepper motors will operate at a lower current than maximum, with a corresponding reduction in torque. You don't want to exceed the maximum current per phase of stepper motor as doing so will damage the motor.
Also, check out Robert's video on how to set the motor current limit by the sound of the motors as the machine operates.