Gap-mode plasmons that occur between metallic nanoparticles and metallic films separated by a thin spacer have been widely studied in the field of nano-optics and plasmonics for enhancing the light-matter interaction of graphene and other two-dimensional (2D) materials. However, efficient photovoltaic devices using such gap-mode plasmons have not been achieved because of structural difficulties. Here, a gap-mode plasmon-induced asymmetric vertical homojunction photovoltaic device using multilayer graphene is presented. In this structure, the multilayer graphene acts both as a photo-carrier generation layer and as a spacer for the gap-mode plasmon. The optical absorption of graphene is further enhanced by the presence of gap-mode plasmons, and the photoresponse time is extremely short because of the atomically short channel lengths across the vertical direction. The wavelength dependence of the gap-mode plasmon is also investigated for three devices with different metal electrodes by photocurrent measurement at five different wavelengths and numerical simulations. The device strategies implemented in this work can enhance the performance of graphene-based vertical photonic devices and can be applied to other 2D materials-based photonic devices.