This paper describes the design, simulation, packaging and measurement of a 10-GHz Gallium Nitride power amplifier (PA). GaN transistors or power cells with maximum-available gain of 8.7 dB at 10 GHz were used in the design. The power cells were manufactured using a 0.8-??m HFET 9-layer process, while the PA's DC bias lines, input and output matching circuits were built on a low-cost Miniature-Hybrid MIC chip. A flip-chip technology was used to integrate the power cells with the rest of the PA circuit. A class-AB PA was designed to provide an output power of up to 39 dBm at 10 GHz, while maintaining a power-added efficiency (PAE) of 31%. The measurements of the PA are presented and issues related to packaging and measurements are also discussed in the paper.