Gallium Nitride (GaN) High-Electron-Mobility Transistors (HEMTs) are emerging as critical components in aerospace power systems due to their superior radiation tolerance and high-frequency capabilities. While radiation effects on high-voltage devices are well documented, the degradation mechanisms of low-voltage p-GaN HEMTs remain less understood. This study investigates the Single Event Effects (SEE) and subsequent electrical degradation of commercial 40 V Schottky-type p-GaN HEMTs under ⁸⁴Kr¹⁸⁺ heavy ion irradiation (LET = 37.9 MeV·cm²/mg). Experimental results indicate a Single Event Burnout (SEB) threshold of 52 V. Following irradiation under a safe off-state bias of 40 V, the device exhibited distinct degradation patterns: the drain-source leakage current (I_ds) increased by over two orders of magnitude, and the threshold voltage (V_th) shifted negatively by 0.26 V. Conversely, the gate leakage current (I_gss) showed an anomalous decrease. Combined TCAD simulations and energy band theory analysis reveal that, unlike high-voltage devices where damage typically occurs in the drift or buffer regions, degradation in these low-voltage devices is concentrated within the gate stack. The proposed mechanism attributes this to irradiation-induced holes accumulating near the gate and being captured by donor-like hole traps located in both the p-GaN layer (deep-level defects at EV + 2.8 eV) and the AlGaN barrier (EV + 0.7 eV). These trapped positive charges lower the electron potential barrier in the channel, causing the negative V_th shift and increased subthreshold leakage. Simultaneously, the compensation of acceptors in the p-GaN layer by these hole traps widens the depletion region of the Schottky junction, resulting in reduced gate leakage. These findings provide essential physical insights for the reliability assessment and radiation-hardening design of low-voltage GaN power modules for space applications.