These N20 6V 70RPM Micro Metal Gear Motor With Encoder is intended for use at 6V. Though in general, these kinds of motors can run at voltages above and below this nominal voltage; so they should comfortably operate in the 3 – 6 V range (rotation can start at voltages as low as 0.5 V).
Lower voltages might not be practical, and higher voltages could start negatively affecting the life of the motor. The micro metal gear motors are available in a wide range of gear ratios—from 5:1 up to 1000:1. With the exception of 1000:1 gear ratio versions, all of the micro metal gear motors have the same physical dimensions; so one version can be easily swapped for another if your design requirements change.