Hybrid plasmonic waveguides (HPWs) are capable of supporting subwavelength optical modes. In a composite HPW (CHPW), the propagation loss can be minimized by adjusting the geometrical parameters of its component layers to reduce field flux inside its lossy metal layer. A ring resonator-based plasmonic sensor based on a waveguide structure of suspended CHPW (SCHPW) is designed for gas sensing applications. SCHPWs are applied for the introduced sensor’s 200-nm-wide bus waveguide and 1-μm-radius ring resonator. The operational parameters of the sensor, such as sensitivity and figure of merit (FOM), are investigated in the near-infrared region using a three-dimensional finite-difference time-domain method. For two considered resonances of the proposed sensor, sensitivities of 236.2 and 270 nm / RIU with FOMs of 67.4 and 37.5 RIU ^{− 1} are achieved, respectively. Additionally, for the proposed sensor, a straightforward mechanism for sensing the mass density of the polarizable hydrogen gas is introduced using the theoretical index–density relation of Lorentz–Lorenz. The mass density sensitivities of 358.2 and 409.3 nm / ( g / cm³ ) are achieved for the two considered resonances for the hydrogen gas at the range of 0 to 0.05 g / cm³.