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Geosynthetic clay liner (GCL) is commonly used as landfill cover and base liner material to limit the transfer of landfill gases and leachates from the landfill to the environment. In the engineered landfill, the gas transfer occurs by advection and diffusion, which may take place at different time frames or simultaneously depending on placement conditions. Hence, the efficacy of GCL as a gas barrier under advection and diffusion needs to be assessed at various geoenvironmental settings. In the present study, the gas flow response of unsaturated GCL under advection and diffusion were evaluated at various apparent degree of saturation, distortion levels, hydrating fluids, and wet-dry cycles using the custom-designed test apparatus. The gas permeability and gas diffusion coefficient of GCL were found to decrease by four and three orders of magnitude respectively with an increase in apparent degree of saturation from 7% to 80%. The gas permeability and gas diffusion coefficient were increased with an increase in distortion levels 0 to 0.169. With an increase in wet-dry cycles 1 to 5, gas permeability and gas diffusion of GCL were increased marginally when hydrated with distilled water while it was increased by one order of magnitude for hydrated with 0.0125 M CaCl2 solution. The results obtained from the present study were compared with published results and found to be well agreement.