The inherent dissipated energy losses (Ediss) during the charging-discharging cycle of output capacitance (CO) in gallium nitride high-electron-mobility transistors (GaN HEMTs) limit their efficiency in soft-switching applications. Accurate characterization of Ediss is important for power device selection and for understanding their origins. Existing measurement methods either fail to apply realistic voltage stress, or might include hard turn-off losses that can result in misleading conclusions. Here, we propose a resonant-Sawyer-Tower (RST) method, that only requires a simple resonant pulse generator and an ST branch to extract Ediss under MHz operations, which was subsequently corroborated with a thermal method. Using the proposed method, we investigated the dependence of Ediss on drain-source peak voltage (Vm), resonant transient frequency (ftr), and switching frequency (fs) for six commercial GaN HEMTs, showing distinct power losses from the CO hysteresis, e.g., from 0.3 W to 1.47 W at Vm = 400 V, ftr = 10 MHz, and fs = 2 MHz. In particular, we discovered a strong dependence of Ediss on fs for some devices, which had previously been overlooked. The proposed method and new findings in this paper are essential for future designs of high-efficiency GaN power converters, especially operating at high switching frequencies.