In December 2018, satellite broadcasting of ultrahigh definition television will begin in Japan. The broadcasting will be provided with an amplitude and phase shift keying (APSK) modulation scheme. The non-linear characteristics of the satellite broadcasting transponders need to be reduced, and to do this, we have been studying a 120-W gallium-nitride (GaN) high power amplifier (HPA) for broadcasting satellites in the Ku band. In this work, we describe a GaN HPA and linearizer (LNZ) we have developed. The HPA was fabricated with $\pmb{0.15-\mu}\mathbf{m}$ GaN high electron mobility transistor (HEMT) technology and delivered an output power of 120 W while maintaining a power added efficiency (PAE) of 31%. The LNZ was applied to the HPA for improving the linearity of the HPA. By applying LNZ, gain compression within 2 dB and phase modulation within 5 degrees were achieved. As a next step, we will perform transmission experiments with a 16APSK signal.