I'm not sure I see how you're correcting the frequency of the gas powered generator short of changing the engine RPM, it's a fixed ratio. If you're saying your VFD is adjusting the engine speed, that's all I'd expect you could do. Obviously you can monitor the frequency and kill the generator, but that's probably not desirable in many cases.
It's a Variable Frequency Drive (VFD). It can take the input frequency and modify to a higher or lower level of any input source (generator or grid). We use them a lot on center pivot irrigation with variable speed motors. Instead of adjusting the voltage of a fixed votage AC motor that is designed only for 480 volts, we lower and raise the frequency of the power to slow down or speed up the motor. VFD's are also commonly seen used with Boiler pump motors to change the flow rate again not by changing the voltage but but changing the frequency of the power going to the motor.