We report an approach for generating an evenly spaced, flat, and spectrally broad optical frequency comb (OFC) by the exploitation of gain modulation experienced by a fourth-ordered periodic Gaussian pulse train. The spectrally flat OFC generator is realized by propagating the periodic optical pulse through high gain SOA. The spectrum flatness limits up to 10 dB for 102 frequency lines, which is improved by exploiting four-wave mixing in 1-m long highly nonlinear fiber. We report a 30-GHz frequency line spaced, 3.06-THz spectrally broad OFC with 2.6-dB maximum power deviation.