The work presents a light weight and compact solid-state power amplifier (SSPA) using GaN technology for L1 band (1.575GHz) GPS applications. Due to immature development of GaN transistor modeling, the PA is designed to operate in Class F mode by using a combination of harmonic load-pull simulations and data-based characterization to achieve high PAE. Additionally, the paper will discuss the solutions to the challenges of thermal management and input matching network design. A hybrid circuit realization achieves Pout=102W, ETA=68% and PAE=66% with gain=15.1dB under 3dB gain compression in CW operation.