In light of recent studies that show efficient particle acceleration and consequent gamma-ray emission in the magnetosphere around the supermassive black holes (SMBHs), we can expect gamma-ray emission from isolated stellar-mass BH (IBH) magnetospheres formed by accretion of interstellar medium (ISM), as well. In this work, we perform one-dimensional general relativistic particle-in-cell simulations of the IBH magnetosphere. We find that a strong electric field intermittently emerges and particles are efficiently accelerated, similarly as SMBH cases. We also build a semi-analytic model of the plasma dynamics and the photon transfer, which can explain the simulation results. Based on this model, we show that the resultant gamma rays are detectable with the Fermi Large Area Telescope and the Cherenkov Telescope Array, when nearby IBHs are embedded in dense gas clouds. Gamma-ray luminosity is strongly dependent on the disk property and the electric current flowing in the magnetosphere.