The wireless power transmission technology using an electro-magnetic resonant coupler (EMRC) has been applied to an isolated direct gate driver for GaN power switching devices. This direct gate driver with the Drive-by-Microwave technologies dose not needs an additional isolated voltage source and a photo-coupler because it can supply an isolated gate signal and signal power all together. The wireless power transmission capability in the driver is crucial for its performances, especially, regarding a switching speed and power consumption. This paper presents the potential of GaN/Sapphire direct gate driver using 5.8GHz wireless power transmission with a compact butterfly-shaped EMRC. Since the fabricated direct gate driver with the integrated EMRC drove a GaN power switching device with a fast turn on/off time, it is proved that the GaN/Sapphire HFETs is best suitable for the direct gate driver.