The transient process of dissipative soliton (DS) generation is numerically revealed in normal dispersion fiber lasers. It is shown that the gain dispersion is critical for the DS generation. The steep spectral edges of DSs are the consequence of the interaction among the normal dispersion, fiber nonlinearity, gain and loss, and gain dispersion effect. Narrow gain bandwidth could result in multiple DSs while broad gain bandwidth (up to 100 nm) could support DS generation provided that enough pump power is available. Consequently, DSs could be generated in fiber lasers even when the gain bandwidth is broad and there is no requirement for other spectral filtering in cavity.