This study presents a tunable dual-band gallium nitride (GaN) power amplifier (PA) operating in L-band. The first band is aimed near the lower edge of the L-band, 1GHz, and the second band is aimed near the upper edge of the L-band, 2GHz, which is located around the second harmonic of the first band. A pin diode-based tunable load matching circuit is proposed and designed in order to present the optimum fundamental and harmonic load impedances to the transistor in both operating bands for maximum efficiency and output power. A method of controlling the tunable load matching circuit according to the input frequency is proposed by the design of a band-selective power detection circuit incorporated into the source matching circuit of the PA. The implemented PA delivers 41.5dBm output power with 81.3% drain efficiency (DE) at 920MHz and 41dBm output power with 60.2% DE at 1720MHz.