The current transport mechanism at metal gate/p-GaN interface in p-GaN HETMs has been investigated. Space Charge Limited Current (SCLC) well describes the behaviour of current density (J_G) at lower applied bias (V $_{\text {G}} \lt 6$ V), while Thermionic Field Emission (TFE) represents the dominant current mechanism at higher V_G. Then, p-GaN gate reliability was investigated by time-to-failure (TTF) analysis carried out at constant positive V_G. In particular, the devices’ lifetime as function of the applied V_G was described considering the J_G-V_G dependence according the TFE model. In this way, a maximum V_G for 10-year lifetime (V $_{\text {Gmax}}^{{10}~\text {years}}$ ) of 8.5 V has been estimated, significantly higher than that extracted by conventional E-model (7 V).