This study demonstrates a simulation model of a 2.8 μm gain-switched Er∶ZBLAN fiber laser by numerical simulation. The simulation results show that there exists an optimal transmittance of output mirror, resulting in an largest output power. The coupling efficiency has significant impacts on the pulse width, peak power, and pulse shape. Accordingly, the influence of the pump power on the pulse shape is calculated and discussed. A signal pulse appears during two pump periods with a weak pump, whereas several signal pulses appear during one pump period with a strong pump. To the best of our knowledge, calculation results, such as the influence of the coupling efficiency on the output power, pulse energy, and pulse shape and the weak signal consistent with pump period when the pump power is around the threshold, have not been reported in any experiment. This study shows that a stable gain-switched pulse output in a 2.8 μm Er∶ZBLAN fiber laser can be achieved only with appropriate coupling efficiency and pump power.