This paper describes a high output power (133W) inverse class-F (class-F-1) GaN HEMT power amplifier (PA) for space applications. The PA operates at 1.227GHz-the L2C second civilian GPS signal operation frequency. Harmonic controlling matching networks are designed to achieve class-F-1 current and voltage waveforms at the intrinsic drain. High efficiency is achieved by using harmonic source and load-pull tuning to design the input and output matching networks. The fo matching is performed using a low-Z single-section transformer. Harmonic terminations for 2fo and 3fo are realized using shunt transmission lines placed at optimal offsets from the drain. These matching networks are fine-tuned using a network analyzer. The overall PA performance was tested under different DC biases with swept input power. The best performance is achieved with a bias of Vds=50V, Vgs=-3V and Pin=32dBm where the PA exhibits 133W output at 42% power-added efficiency (PAE).