This paper describes the design and evaluation of a single stage 125W L-Band GaN Power Amplifier using a low-cost packaged transistor. The amplifier is optimized for the 0.96 to 1.215 GHz band with a typical band center gain of 20dB and output power at P-3dB of +51dBm (125W) at a PAE (for the connectorised amplifier) of > 70%. The design is based on a commercially available discrete 0.25μm GaN transistor, housed in a metal-based ceramic package (the QPD1008 from Qorvo). The amplifier uses 32mil thick Rogers 4360G2 PCB material mounted on an aluminium carrier.