We investigated the breakdown mechanism and distribution of the reverse gate leakage in Gallium Nitride High Electron Mobility Transistors. The devices show a hard and high breakdown of several tens of volts under deep OFF-state gate bias but much less soft breakdown of just a few tens of volts for near-threshold OFF-state conditions. We establish that the low and soft breakdown is due to space-charge-limited-current from drain to source. Using numerical simulations of practical device structures, we show that, while both field plate and high-k passivation techniques can be equally effective at higher breakdown voltages, the high-k passivation is less effective at lower breakdown voltages. Simulation based prior works have highlighted the role of gate edge effects on reverse gate leakage. Still, prior models have assumed it to be predominantly areal. For the first time, we extract the components of gate leakage flowing into the edge and area of the gate, from the measured data of devices with different gated and un-gated lengths. We find that, in short channel devices, most of the gate leakage flows into the edge. Hence, one-dimensional gate leakage models will not work for simulating short channel devices, though such models have justifiably been used to discern the gate leakage mechanism using large area devices.