Disposal of radioactive waste in deep geological repositories relies on the integrity of geological barriers, where gas migration can compromise long-term safety. This study examines the hydro-mechanical response of a shale under varying gas pressure build-up rates, using gas injection tests conducted in a high-pressure oedometer cell to simulate in-situ stress conditions. The research highlights that gas-induced porewater redistribution plays a key role during gas invasion processes. Results indicate that rapid gas pressure build-up leads to undrained conditions associated with significant porewater pressure development and expansive strains, while slower gas injection results in a drained response with less deformation. Additionally, a delayed gas breakthrough during rapid pressure build-up suggests the impedance of gas movement by porewater. However, once steady-state is achieved, both tests converge to similar gas flow rates and equilibrium states, indicating that the long-term gas transport properties of Opalinus Clay, selected as the host geomaterial for the Swiss repository, are not significantly influenced by initial gas pressure rates. Furthermore, neither the water intrinsic permeability nor the pore size distribution of the material is altered by gas invasion, highlighting the robustness of Opalinus Clay as a geological barrier for radioactive waste disposal. These findings emphasize the importance of understanding both short-term and long-term hydro-mechanical responses of shales subjected to gas transport to ensure the long-term containment and isolation of radioactive waste.