Slot waveguide has emerged as a potential candidate for the design of evanescent field absorption based photonic gas sensors, optical quantum information applications, etc. In this paper, three different slot waveguide structures, i.e., conventional slot, partial-strip-loaded slot, and full-strip-loaded slot waveguides have been explored to analyze their sensing performance for the methane gas. As the methane gas has the peak absorption at around 3.31 µm, therefore, this has been considered as the operating wavelength for the analysis. In anticipation of improvement in evanescent field ratio in slot region and hence, the sensing capabilities of the gas sensor, the slot waveguide structures have been designed by depositing the germanium layer over the calcium fluoride in different manners. To realize the significant evanescent field and sensitivity along with relatively low propagation loss, the suitable dimension of the slot waveguide structures has been chosen very judiciously. Several waveguide parameters, such as evanescent field ratio, propagation loss, and sensitivity have been chosen for the analysis and comparison of slot waveguide structures, by varying the arm-width and thickness of germanium layer. Simulation results have demonstrated that the full-striploaded slot waveguide has the superior performance in terms of higher evanescent field and higher sensitivity, which is followed by the partial-strip-loaded slot waveguide, even for the fixed target value of propagation loss. Moreover, the current analysis may be extended for the design of suitable photodetector that can further enhance the performance of the gas sensor.