MOSFET gate protection zener diode math .please help
If the specified gate threshold voltage is (say) 2 volts this doesn't mean the FET is turned on like a relay contact when you apply that gate-source voltage. There are graphs in the data sheet that will show you (typically) what sort of gate drive you need for a certain gain current with a certain volt drop across drain to source. Vgs(threshold) is just a single parameter and certainly does not guarantee reasonable on-resistance.

You should look at the graphs, estimate the gate-source voltage needed then get a zener that is slightly higher than this voltage but, below the 8V limit. Provide a data sheet link and I'll show you the relevant graph.