Surface plasmon polaritons (SPPs) are waves that result from an intimate interaction between electromagnetic waves and metallic surface electrons. The permittivity of the metallic material that supports SPPs in a surface plasmon gap waveguide (SPGW) has a complex value in the optical frequency. The size of the waveguide cross-section that gives rise to this cutoff condition can be significantly decreased in the case of a SPP rectangular waveguide. The SPGW can create optical fields that are strongly confined and strongly enhanced in the nanosize gap region and can guide these fields along the nanosize gap structures. The phase velocity of the SPP propagating along the waveguide can be controlled by the gap width of the SPGW. The spot size of the incident Gaussian beam is sufficiently smaller than the cross-section of the metallic screen that negligibly small guided waves are excited by the screen into the straight SPGW by the incident field.