Gallium nitride (GaN) technologies have become an essential role in commercial advanced RF systems, which accompany emerging RF electrostatic discharge (ESD) reliability challenges. As opposed to ESD clamp transistors in LV CMOS technologies, a mis-correlation between standard-defined human body model (HBM) ESD robustness and commonly used TLP failure current was observed in GaN (MIS) high electron mobility transistors (HEMTs). Using transient HBM I-V characteristics, a novel discharge model is proposed to explain the transient discharge mechanism. The TCAD and SPICE simulations confirmed that the observed mis-correlation between TLP and HBM is attributed to 2-dimensional electron gas (2DEG) resistance modulation in response to HBM ESD transient voltage waveforms. The HBM waveforms under full transient duration in terms of rising and falling edges are further discussed. Eventually, the failure mechanisms in the TLP IVs and the HBM transient IVs can be well correlated in GaN (MIS)HEMTs.