A picosecond erbium-doped fiber laser based on gain-switched laser diode is demonstrated. The gain-switched laser diode seed can produce 80 ps pulses with tunable repetition rates from 100 kHz to 20 MHz. After master-oscillator power-amplifier (MOPA), this fiber laser yields 9.3 W average power at 20 MHz repetition rate, corresponding to 5.8 kW peak power.