High-density bentonite pellets mixed with powdered bentonite are promising sealing materials for deep geological disposal of high-level radioactive waste (HLRW). Gas migration in such systems is complex due to the double-porosity structure, with macrostructural pores between pellets and powder particles and microstructural pores within the clay particles. This study presents a three-dimensional coupled hydromechanical model of the BENTOGAZ_1 gas injection experiment, which was used to analyze the simulated coupled multiphase flow hydromechanical behavior during hydration and gas injection in an MX-80 bentonite mixture composed of 50% pellets and 50% powder. The Barcelona Expansive Model (BExM) was employed to simulate double-porosity interactions under coupled hydraulic and mechanical loading, with particular attention to the influence of selected constitutive parameters on the predicted response. The model captures key experimental trends during both hydration and gas injection, including axial stress, liquid pressure, and gas pressure evolution. A parametric sensitivity analysis was performed on a selected set of the main constitutive parameters of the BExM. These parameters, including the macrostructural and microstructural elastic compressibility parameters and the preconsolidation pressure, were analyzed to evaluate their influence on the predicted coupled hydromechanical response of the pellet/powder mixture. Among the compressibility parameters, the microstructural compressibility exerts the strongest influence because it dictates how wetting-induced strain is partitioned between the microstructural and macrostructural domains, thereby controlling macropore connectivity, permeability evolution, and gas transmission. The influence of the macrostructural parameters is comparatively modest. Variations in the macrostructural elastic compressibility modify the stress and suction path mainly when it increases rather than decreases, however their impact on gas transport remains secondary to that of the microstructural compressibility. These findings reinforce the importance of characterizing strain redistribution in pellet/powder double-porosity structure for reliable barrier performance prediction.