What is the purpose of diode in parallel with the resistor divider in MOSFET gate driving circuit
Given R3, R4 &amp; C1 that diode is doing mostly diddly squat. 

Generally you add a diode in parallel with the driving resistor at a gate of a Mosfet to accelerate turn-on or turn-off by providing more drive to the Miller-multiplied Drain-Gate capacitance. 

In this particular case the diode seems to be there to bypass the resistive divider and provide more voltage drive to the gates. Which is then wasted in the other resistors and capacitors. 

What they might be doing is to provide for a relatively slow turn-off while not compromising too much on turn-on time. Perhaps it’s part of a zero-crossing turn-off circuit?