Hz is a function of rpm. so either the engine is running slow or the gauge isn't reading correctly.
Yes and no. Yes, hertz =RPM. But on this set, and most others, there is a A7 (Frequency Transducer), that gives the hertz meter its signal. It takes 120 volts, (when the set is running right) and converts that to a DC signal the M2 meter can read as 60 hertz. If the AC voltage is low, or there is maybe only excitation voltage coming out of the main gen, then the A7 will not read right. Hence, low frequency.
You did reset the A2 (Fault Indicator)?
Have you downloaded the TM's and tried to apply the troubleshooting guide?