In this paper, we investigate a 1018 nm gain-switched ytterbium-doped fiber oscillator at a low repetition rate in terms of theory and experiment. Theoretically, a numerical model applicable to a 1018 nm gain-switched ytterbium-doped fiber laser was established. The influence of the pump peak power and active fiber lengths on the 1018 nm gain-switched ytterbium-doped fiber laser was numerically simulated. Experimentally, a compact 1018 nm all-fiber-structured pulsed laser oscillator is constructed, in which a pulse width of 110 ns and a single-pulse energy of 0.1 mJ were obtained. Moreover, the experimental results are in agreement with the numerical simulation ones. To the best of our knowledge, this is the first time that gain-switching technology has been applied to 1018 nm fiber lasers to generate nanosecond pulsed lasers. The model and experimental results can provide a reference for the engineering design of the same type of low repetition rate fiber lasers below the kilohertz level.