The effects of gain nonlinearities on gain-switched short-pulse-generation characteristics are analyzed via rate equations assuming a nonlinear-gain model including a gain saturation parameter gs to quantitatively describe the strong gain-saturation nonlinearity in low-dimensional semiconductor lasers at high carrier densities. It was found that the minimum pulse width and the delay time are mainly determined by gs rather than a differential gain coefficient g0 and a gain compression factor ε. By tracing the temporal evolution of carrier density, photon density, and material gain during gain switching, distinctly different effects of gs, ε, and cavity lifetime τp on pulse generation were clarified.