The resistance shown is not M\$\Omega\$, try 9\$\Omega\$ (read the datasheet).
Anything much more than 5-10\$\Omega\$ would slow the switching excessively for many applications (and they are wanting to sell that to everyone possible). M\$\Omega\$ would slow the switching to many milliseconds not hundreds of ns.
A large value resistor will not likely protect the gate anyway, it's a permanent breakdown and insulation damage that occurs, not like a diode breakdown. That's why the ESD zener diodes are on the gate lead, to prevent excessive gate-source voltage.
So, why put any resistor at all in there you ask? Well it's so the other (Overvoltage) zeners can do their thing. Imagine the worst case and we short the gate lead to the source, and then sadistically increase the voltage on the drain (through some external load) waiting for the D-S breakdown. When the current through the zener diodes exceeds a couple A the MOSFET turns on and clamps the overvoltage. If the current is less than that, the zener + 9 ohms clamps it through the gate (possibly affecting whatever is connected to the gate! ).