Gallium Nitride has shown its potential over traditional Silicon devices with low switching and conduction losses. However, fast switching transition poses great challenges in dynamic device characterization due to sensitivity to parasitics and high requirement in testing equipment. An accurate switching behavior characterization method is proposed in this paper with loop inductance minimization and flat current shunt resistors. Switching losses for GaN transistor under three different current freewheeling patterns are explored and compared: through upper diode, through upper GaN transistor synchronous operation and through upper GaN reverse conduction, which include all the possible operation conditions in practical industry applications. The overall inductance in main power loop is well designed as low as 1.52nH. It is the first cleanest and most accurate GaN switching waveform ever reported. It is also the first time that GaN switching behavior with current reverse conduction through channel is investigated. The fastest switching speeds are dv/dton = 9.1V/ns, dv/dtoff = 8.6V/ns, and di/dton=7.0A/ns, di/dtoff = 5V/ns. The measured turn-on energy loss is 1.4μJ at 30V, 20A and 1.59μJ at 30V, 30A respectively.