The paper defined the laser-induced damage threshold from the fluence and the peak power of GaSe and GaSe:In single crystals upon exposure to nanosecond radiation in the two micron range and assessed the influence of test radiation energy parameters (pulse repetition rate, pulse duration) on the damage threshold. Laser-induced damage threshold was determined with the parameters of the incident radiation close to the pump radiation parameters of promising dual-wavelength optical parametric oscillators (effective pump sources for THz difference frequency oscillators): wavelength was ≈2.1 μm; pulse repetition rates were 10, 12, 14, and 20 kHz; and pulse durations were 15, 18, 20, and 22 ns. The obtained results made it possible to conclude that the value of GaSe damage threshold at a wavelength of 2.091 μm of the incident radiation was influenced by the accumulation effects (the damage threshold decreased as the pulse repetition rate increased). The accumulation effects were more significant in the case of the In-doped sample, since a more significant decrease in the damage threshold was observed with increasing frequency in terms of the peak power and the fluence.