Dynamic ON-resistance (RON) in GaN HEMTs is a critical issue for power electronics efficiency. This paper proposes a novel method to measure dynamic (RON) under long-term OFF-state drain-stress and steady-state soft-switching, comparing Schottky-type (GaN-1) and Ohmic-type (GaN-2) p-GaN gate HEMTs. Results reveal significant dynamic (RON) increases after long-term OFF-state stress, e.g., 1.8× for GaN-1 and 3.3× for GaN-2 at 100 V for 1 min, with non-monotonic voltage dependence and long stabilization times (>1min), highlighting OFF-state trapping effects in both devices. However, distinct de-trapping behaviors emerge: GaN-1 shows only partial (RON) recovery during ON-time, while GaN-2 achieves near-complete recovery within 150 μs due to hole injection from p-GaN regions during switching, with <10% residual increase at steady state. We found that for GaN-2, gate injection effectively mitigates dynamic (RON) increase under soft-switching, while additional hole injection from drain-slde p-GaN accelerates de-trapping in hard-switching. These findings highlight the role of OFF-state trapping and hole injection, providing critical insights for optimizing GaN HEMTs in power applications.