Gas breakthrough pressure is of great importance for evaluating the sealing efficiency and assessing the long-term performance of saturated buffering materials with low permeability in the deep geological disposal of high-level radioactive waste. In this study, to investigate gas breakthrough properties, step-by-step (SBS), residual capillary pressure, and mercury intrusion porosimetry tests were conducted on saturated Gaomiazoi (GMZ) bentonite specimens. Results show that, compared with the other two methods, only SBS tests can accurately measure the gas breakthrough pressure. Meanwhile, the results of gas injection tests by the SBS method indicate that gas intrusion into a specimen in the inlet accompanied by water displacement at the outlet is not an instantaneous process. Therefore, the time required for pore pressure equilibrium during each pressure step was calculated, which is in inverse proportion to the intrinsic permeability of the compacted bentonite materials. Furthermore, the gas injection tests also show that gas breakthrough could also occur at lower pressure levels, i.e., snap-off pressure, which is far below the value of the breakthrough pressure. The differences between the breakthrough pressure and the snap-off pressure could be induced by the hysteretic behavior of the bentonite material during the drainage and imbibition processes. The delayed re-imbibition of water will induce further gas migration through the interconnected pore space and a lower capillary pressure. Experimental results in this work are very useful for the engineering design and safety assessments of the repository.