Quantum emitters with high emission rates and efficiently coupled to optical waveguides are in demand for various applications in quantum information technologies. Accurate positioning of a quantum emitter within a strongly confined gap-plasmon waveguide (GPW) mode would allow one to significantly enhance the decay rate and efficiency of channeling of emitted photons into the waveguide mode. Here, we present experimental results on the GPW mode excitation in a gap between a monocrystalline silver nanowire and a monocrystalline silver flake by using a single nitrogen-vacancy center in a nanodiamond. The coupled systems containing a nanodiamond and the structure supporting the GPW mode are created by a combination of electron beam lithography and nanomanipulation with an atomic force microscope (AFM). In these systems, we find decay rate enhancements of up to ∼50 and the efficiency of channeling of photons into the GPW mode of up to 82%, resulting in an exceptionally high figure-of-merit of 212 for the emitter-plasmonic waveguide coupled system. The results obtained suggest new avenues for practical realization of integrated solid-state quantum systems.