A Class F power amplifier (PA) operating in the 2.5-2.7 GHz frequency band has been designed, fabricated, and tested using a gallium nitride high electron mobility transistor (GaN HEMT). The amplifier has a peak power-added efficiency (PAE) of 76.4% and output power of 8.5W with a 12.3 dB gain. This combination of high gain and high-efficiency operation across a wide frequency range creates opportunity for the design's use in various commercial and defense applications where power consumption is a major design factor. Advanced design system (ADS) from Keysight is used for the design and simulation process. The input matching network and harmonic termination circuits are designed using the reflection data of the transistor which is extracted using non-linear model over harmonic balance analysis. The output matching circuit is designed using the obtained load pull data after the harmonic termination network design is completed. The design procedure is presented in detail, the input and output matching circuits are made according to a specific method and order. The design based on the usage of reflection data is obtained under large signal conditions. These data are then defined and evaluated as a network termination in the ADS tool. This termination hence enabled us to design harmonic suppression and output matching networks regarding the output impedance of the transistor. While design and optimization is not an easy task for non-linear analysis, it is observed that the followed and proposed procedure makes such designs easier and practical with successful results.