Various dissolved gases, such as CO2, H2, and CH4, may be present in the near-field geological repository due to metal corrosion or the degradation of organic waste. However, the influence of dissolved gases on the swelling behavior of bentonites, commonly used as backfill material, is still poorly understood. In this study, classical molecular dynamics simulations are conducted to investigate the swelling behavior of Na-Mt as a function of compaction and the presence of dissolved gas. The simulations revealed that the presence of gas molecules increases the swelling pressure of Na-Mt and can be quantitatively explained by Henry’s law, which describes the increase in the gas solubility with pressure. The magnitude of the effect is gas-specific, depending on the dry density of clay and the gas concentration in the external reservoir. Accordingly, a general equation describing the enhanced swelling pressure due to gas solubility is proposed. A detailed analysis of structural transformations in the clay interlayer provided a model-based explanation for the discrepancy between the experimentally measured swelling curves and the simulated curves at high compaction levels. These findings contribute to a better understanding of the fundamental mechanisms underlying the swelling behavior of clays used as barrier material in deep geological disposal.