The PWM carrier frequency is about 7.8kHz. It uses one of the 8-bit timers and has 255 output voltage levels 5/255 V for the min rpm and 5V for the max rpm. 0V for disabled.
FWIW, I’ll be releasing a new Grbl v1.0 beta on Github later this week (in the edge branch). This will include ‘$’ settings for max and min rpm to help you dial in the rpm with Grbl’s voltage output signal.