Gain-switched semiconductor lasers can produce pulses with naturally randomized phase, which makes them a convenient light source for quantum key distribution and random number generation. Nevertheless, semiconductor  lasers are vulnerable to external optical feedback, a phenomenon, characterized by injection of a certain part of laser radiation into the laser’s diode cavity. Although optical feedback may be used to decrease relaxation oscillations and chirp, it may have negative effect on laser pulses. Here, we study the influence of optical feedback on the pulse shape of a gain-switched laser.