This paper presents the influence of gas bubbles trapped in a soil mass on the stress–strain–strength response of medium-dense sands. A hypoplastic constitutive sand model enhanced with the intergranular strain concept was coupled with the Fredlund gas–fluid compressibility relationship to capture gassy soil behavior. Boundary value element representations in a finite element platform of oedometer and saturated drained and undrained triaxial compression tests are performed for the calibration of soil parameters. For the numerical simulation of gassy soil behavior, pore fluid compressibility is modified to account for the presence of free gas in the pore fluid. The gassy soil mechanical response is studied by using only one set of parameters determined from the saturated soil response. The testing bed for this evaluation is a laboratory experimental program conducted on sands retrieved from the Oakridge Landfill, a sanitary landfill located in South Carolina, USA. The hypoplasticity sand model specialized with the Fredlund relationship reproduces reasonably well the stress–strain–strength response of these sands for a wide range of loading conditions and at a reasonable level of testing for the calibration of constitutive parameters. It is found that a slight reduction in the degree of saturation significantly decreases the undrained shear strength of the soil and causes changes in volume (i.e., drained-like behavior).