Commercially available normally-off GaN power HEMT devices have typically adopted a p-GaN gate structure. In the gate region, there exist a Schottky junction and a p-GaN/AlGaN/GaN heterojunction. With drain voltage changing during a switching process, the variation of net charge in the floating p-GaN layer would cause instability in threshold voltage. Besides, the gate-related junction capacitances would exhibit behavior different from those of a Si MOSFET. Consequently, the switching transient performance of a GaN transistor with a p-GaN gate could be significantly influenced by the aforementioned factors. In this work, the threshold voltage instability associated with the drain-to-gate voltage stress is firstly analyzed. Despite the difficulties in directly performing capacitance measurements inside the device structure, a hybrid physical-behavior modeling method is proposed. The model is capable of extracting the capacitance-bias relationships with regard to the gate region from static terminal measurements. In previous works, the advanced analytical model would make modest change on Si MOSFET's model. As a result, the simulated waveforms would exhibit 20$\times$50% discrepancy from experiment. In contrast, the proposed switching transient analytical approach would exhibit improved accuracy. Consequently, the switching transient performance of GaN transistor with p-GaN gate could be more accurately evaluated.