The gate reliability of p-GaN gate AlGaN/GaN high electron mobility transistors with a ring-gate structure is investigated by step-stress experiments under a gate bias, combined with electroluminescence imaging and pulsed measurements. For gate stress ( ${V} _{\text {Gstress}}$ ) from 0 to 13 V, the drain current is found to be stable and decreases at 13 V at OFF-state and increases at ON-state. For ${V} _{\text {Gstress}}$ from 13 to 31 V, the drain current increases at OFF-state and decreases at ON-state, whereas ${V} _{\text {TH}}$ is stable and increases slightly. The changes in drain current characteristic and ${V} _{\text {TH}}$ values of the device after applying various ${V} _{\text {Gstress}}$ are associated with the injection and trapping of holes from the p-GaN layer to the AlGaN layer, supported by the results of pulsed measurements and by the simulation of energy band diagrams. When ${V} _{\text {Gstress}}$ is further increased to a high voltage of 31 V, ${V} _{\text {TH}}$ becomes noisy and a strong luminescence signal occurs with a sudden reduction in the drain current, and finally, a catastrophic failure happens in the gate-channel region.