Edit: Re-interpreting the value shown on the datasheet. The resistance shown is not M\$\Omega\$, much lower, more like **3400 ohms** based on the change in the switching times with external gate resistor. 

This does in fact really slow the switching when the gate charge is high, such as the 1.6ms minimum off switching time with a 15V 1.5A load. The asymmetric switching time implies they may actually have a diode across the resistor to speed the 'on' time. The diode will be reverse-biased when clamping, as explained below. 

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 some mA the MOSFET turns on and clamps the overvoltage. 

Power MOSFETs are generally not very sensitive to ESD anyway, because of the large gate capacitance. The gate actually breaks down at something like 50V-100V typically so a lot of energy has to reach the gate. Tiny MOSFETs such as RF MOSFETs are very sensitive to ESD in comparison. However, the typical human body model of ESD is enough to damage even a moderately large power MOSFET gate.