An attractive alternative to generate tunable dual-wavelength pulses by external injection seeding of a gain-switched Fabry-Perot laser diode (FP-LD) is demonstrated. The external injection seeding branch consists of a FP-LD, a 3dB photo-coupler and two fiber Bragg gratings (FBG's). The dual wavelength can be selected and their spacing can be tuned by adjusting two FBG's. The side-mode-suppression-ratio (SMSR) is better than 17dB over a 10nm wavelength tuning range. The whole system is simple and costs lower.