Two-dimensional (2D) materials present various extraordinary properties that are advantageous in optoelectronic devices with atomically thin nature. Despite their excellent light-matter interaction, a low optical absorption that is proportional to thickness is considered to be a major limitation. In this study, a gap-mode plasmon structure is applied to the Schottky junction of Au-MoS2 to compensate for its low absorption. The magnitude of the gap-mode plasmon is generally known to be inversely proportional to the gap distance between two metal nanostructures; hence, an atomically thin 2D material can be considered to be a good candidate for a gap spacer. Owing to the gap-mode plasmon structure, the photoresponsivity of the proposed device is enhanced by approximately 11.6 times from 25 to 290 A W-1 under 1 nW of laser power, without photoresponse time degradation. Two operation modes, named the photovoltaic and the photoconductive mode, are also observed through different response times; these present different carrier transport mechanisms depending on the existence of bias voltage.