A wavelength tunable optical comb is generated based on the gain-switching of an externally seeded Fabry-Pérot laser diode. The comb consists of about eight clearly resolved 10GHz coherent sidebands within 3dB spectral envelope peak and is tunable over the entire C-band (1530 to 1570nm). The optical linewidth of the individual comb tones is measured to be lower than 100kHz, and the RIN of the individually filtered comb tones (←120dB/Hz) is shown to be comparable to the entire unfiltered comb (← 135dB/Hz). Besides, expansion of the tunable gain switched comb is achieved with the aid of an optical phase modulator, resulting in near doubling of the number of comb tones. © 2011 Optical Society of America.