Hi,
The zener protects the gate from excessive voltage. The MosFet Gate to Source max is 20V, So an 18V zener will be fine. One might argue that 18V is kind of close to the 20V max (i.e. not enough safety margin, but I know the 20V is conservative).
When you switch a MosFet quickly, you will need more current to change the gate quickly from 0 to >6 to 8 V. This is because there is an electrical charge in the MosFet (kind of like a capacitor) that must be charged to turn on and discharged to turn off **. What PWM frequency do you want to accommodate?
** I took some liberties to make the concept simple.