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For the first time, we use electrical characterization, spectrally-resolved electroluminescence measurements and degradation tests to explain the negative activation energy of gate reliability in power GaN HEMTs with p-GaN Schottky gate. First, the origin of gate leakage current was modeled. The results indicate that the gate leakage current originates from three different mechanisms: (i) thermionic emission of electrons from the uid-GaN layer across the AlGaN barrier, for gate voltages below threshold $(V_{G} \lt V_{TH})$ , (ii) thermionic emission of electrons from the channel to the p-GaN layer $(V_{TH} \lt V_{G} \lt 4.5 V)$ and (iii) trap-assisted-tunneling of holes at the Schottky metal for higher gate voltages. Then, the analysis of the reliability as function of gate bias demonstrated a negative activation energy (longer lifetime at high temperature). By analyzing the electroluminescence spectra under high positive bias, the improved time to failure at high temperatures was ascribed to the increased hole injection and recombination, that reduces the overall number of electrons that undergo avalanche multiplication, leading to the breakdown. Finally, the model was validated by comparing the electrical properties and conduction model of the devices pre- and post-stress.