A high-frequency a high-power GaN HEMT was analyzed using our full band Cellular Monte Carlo (CMC) simulator, in order to extract small signal parameters and figures of merit, and to correlate them to carrier dynamics and distribution inside the device. A complete RF and DC characterization of the device was performed using experimental data to calibrate the few adjustable parameters of the simulator. Then, gate-related capacitances, such as Cg, Cgd, and Cgs, were directly and indirectly extracted combining small-signal analysis and DC characterization.