Gas migration in low-permeability materials has become a critical focus in energy and environmental geotechnics, particularly for managing the deep geological disposal of radioactive waste. Such waste can release radionuclides over hundreds of thousands of years, making it vital to understand the long-term processes occurring in repositories. Among these, significant gas volume generation, accumulation, and release are of great concern. While these processes vary depending on waste type and repository design, they must be addressed in all safety assessments. To this end, an experimental program was developed to investigate how excessive gas pressures induce damage in clay-rich geomaterials using materials from artificial (granular bentonite) and natural (argillaceous rock) barriers. Evidence suggests that gas preferentially flows through preferential pathways. Microstructural analyses conducted before and after gas injection provide insights into failure mechanisms, including the pore and fracture network geometric descriptors in intact and damaged materials. The self-sealing capacity of these materials was also evaluated using the same gas injection setup. A re-saturation stage assesses water conductivity to quantify the potential loss of its hydraulic barrier function. These findings contribute to the safety case for repositories by improving our understanding of gas-induced damage and the resilience of engineered barriers.