A lot of the commercial/farm PTO generators have gearing built in. Some are made with belt transmissions so the rpm from the PTO could be matched easier than with a direct gear drive. they change the size of the pulleys.
You should still be matching the RPM to get the frequency right on most of them. Voltage and current are usually handled electronically now with the tractor governor just handling the RPM and the fuel to keep it steady under different loads.
Some gennies use an electronic signal from the generator to control the motor speed.
You might find some other information about them on this page.
http://www.omafra.gov.on.ca/english/engineer/farm_mech.htm