UC3843 Boost Converter Circuit
Simple, the power is \$P=RI^2\$, where I is the gate current of the MOS. But how much current enter in the gate? ;-)

The Application Note say that R is for damping, for have a dissipation element of the oscillations that could be occurs in the circuit formed by gate capacitance and wiring incuctance. If your circuit is well-designed you could use a very small signal resistor (0.25W could be enough).