In this article, we illustrate the impact of the high electric field region and the effects of this has on the capacitance-voltage characteristics of a GaN HEMT device. Such effects arise due to a significant spike in the electric field near the drain-side edge of the gate electrode. The presence of the high electric field has a direct impact on the intrinsic capacitances of the device. We present a physics-based compact model for the gate charge and intrinsic capacitances, which captures these effects. TCAD simulations are performed to analyze the underlying principles of such phenomena while providing validation for the proposed model. The advanced SPICE model for GaN HEMTs (ASM-GaN-HEMT) is adopted and modified to include the effects of the high electric field region. The simulation results are in excellent agreement to the TCAD and the measured data. The model is compared to simulations that neglect such effects in order to illustrate the importance of capturing the impact of high-field region on the intrinsic device capacitances.