For synchronous generators, it is simple.
If the gen head is a two pole, it needs to go 3600 to give 60Hz.
If the gen head is a four pole, it needs to go 1800 to give 60Hz.
..............six pole..................1200.......................................
A MEP-016B head, for example, has 3 two pole windings and can therefore give 3 phase 60Hz power at 3600RPM. 3 two pole windings = six wires.
A "12 Wire" gen head has 3 four pole windings. The "12 Wire" heads are good because you can connect the 12 wires in different ways to get different outputs, like single or 3 phase.
You have the frequency gauge because different loads and conditions (temp, altitude, etc.) affect the engine RPM. You can set the engine speed to give 60Hz (1800RPM) at sea level and then move to Denver and you'd be off. Most 120V things will run on anywhere from about 108 to about 135 Volts. Cheaper generators don't adjust. Very expensive gens adjust speed automatically. MEP gen sets give you the ability to manually adjust Voltage and Frequency to get good 120V 60Hz output.