Physics-based compact models have well-known advantages over empirical modeling approaches, such as consistent and physical behavior of the model for different types of device characteristics, continuous capacitance-voltage (C-V) and current-voltage (I-V) behavior for all possible bias conditions, accurate scaling of various currents and capacitances for various process parameters and geometries, etc. The advanced SPICE model for GaN devices (ASM-GaN) is a physics-based model, which uses surface-potential-based drift-diffusion transport equations. Such models require device geometry and process parameters as inputs, which are not typically available for commercial GaN power devices. In this article, for the first time, we have developed a formulation of the ASM-GaN compact model, which does not need device geometry and process details, but still captures the intrinsic FET operation in all regions of device operation. The parameters of this new intrinsic model can be extracted from a systematic set of dc I-V and C-V measurements. Additionally, commercial devices come in packaged form; thus, we have developed a methodology based on off-state S-parameter measurements to capture the effects of the package using a lumped element approach. The resulting model shows good agreement with measured device characteristics of a commercial GaN power device.