We review our state-of-the-art GaN-based device technologies for power switching at low frequencies and high frequency operation aiming at future millimeter-wave communication systems. These two applications are emerging in addition to the widely investigated power amplifiers at microwave frequencies for cellular base stations. As for the power switching GaN devices, we present a novel device structure called Gate Injection Transistors (GIT), which enables normally-off operation with high drain current. Here we also present the world highest breakdown voltage of 10400V in AlGaN/GaN HFETs. In the last part of this paper, we present GaN-based MIS-HFETs which exhibits as high fnax as 203GHz. The successful integration of low-loss microstrip lines with via-holes onto sapphire enables compact 3-stage K-band amplifier MMIC of which a small-signal gain is as high as 22dB at 26GHz with a 3dB bandwidth of 25-29GHz. The presented devices are promising for the two emerging future applications demonstrating high enough potential of GaN-based transistors.