Math behind Zener resistor for MOSFET Vgs
You can find the answer by reading the datasheet.

3 Watt Plastic
Surface Mount
Zener Voltage Regulators

MB5927B
Vz=12V 5% @ 31 mA   = Izt
Zzt = 6.5 Ohms @ Izt
Zzk=550 Ohms @ 0.25mA = 137 mV


  I assert ...   the drop from “threshold,zt to knee zk” can be crudely estimated as a fixed bulk series R.  


Find Threshold Vth or Vzk at the zk “ Zener knee”.
Vz = Vzk(@ Izk=0.25mA) +Izt * Zzt
  ... = 12V 5% = Vzk + 31mA * 6.5 Ohms.
Vth = 12V +/-0.6 - 0.2V = 11.8V +/-?

In actuality Rs rises from Zzt to Zzk or 6.5 to 550 Ohms from Iz = 31 mA to 0.25mA but normally this model works well down to 10% or 3.1 mA.

Conclusion

Thus the FET gate voltage is protected with slightly less voltage limit than the 12V rating.

From datasheet.
  Zener Voltage Regulator
Symbol
Parameter


VZ
Reverse Zener Voltage @ IZT
IZT
Reverse Current
ZZT
Maximum Zener Impedance @ IZT
IZK
Reverse Current
ZZK
Maximum Zener Impedance @ IZK
IR
Reverse Leakage Current @ VR
VR
Reverse Voltage
IF
Forward Current
VF
Forward Voltage @ IF
IZM
Maximum DC Zener Current