We investigate the dispersion properties of an ultrathin spoof plasmonic waveguide based on metal strip grooves using the finite element method. The confinement ability of the surface plasmon polariton (SPP) waves are influenced by the dispersion curves, which can be modulated by the structure parameters. The propagation characteristics of a subwavelength planar plasmonic waveguide ring resonator have also been studied. Furthermore, a gain medium is introduced to compensate for the propagation loss of the SPP wave in the device at terahertz frequency. It is demonstrated that the gain medium provides an enhancement for the control of on/off states of the signal with the presence of pumping, which paves a way for gain-assisted switching and lasing applications in the terahertz regime.