All generators that produce AC power try to stick to 60Hz (or 50Hz, depending on where you live) in order to match both the mains voltage and frequency.
Older (non-inverter) gas generators accomplish this by running the engine at a fixed speed, which spins the generator (technically an alternator since it makes AC) at a constant speed. The alternator is designed to output 60Hz at that fixed speed. The engine has a govenor on it that maintains the fixed speed independent of the load on the alternator (which is load on the engine).
Inverter-type generators are similar, but do away with the need to run at a fixed speed, instead using electronics to create a 60Hz output at all times. When lightly loaded electrically, the engine runs slower, saving gas and minimizing noise. When heavily loaded, the engine runs faster.
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