Measurement results of the terminal capacitances of a high-voltage power GaN high-electron-mobility transistor on a conductive-Si substrate are presented. These results show significant dependence of these capacitances on the substrate (or bulk/backside) voltage. In this article, we enhance the ASM-GaN compact model, which is a recently selected industry standard model for GaN devices, to account for this dependence. A detailed description of the modeling procedure is presented. Simulation results based on the enhanced model are in excellent agreement with measurement results. The model enables the design of advanced high-voltage GaN power ICs, such as half-bridges with drivers and logic, on a conductive Si-substrate, taking capacitive substrate coupling into account during simulations.