Recently, graphene oxide (GO) has been investigated as a class of molecular filters for selective gas and ion transport. However, detailed transport mechanisms have been poorly understood thus far. Here, we report the gas transport behavior of noninterlocked GO and reduced GO (rGO) membranes, which contain nanoporous gas diffusion channels generated by the adjacent edges of GO and rGO sheets. Both membranes exhibited Knudsen gas diffusion behavior; however, the separation factors of these membranes exceeded the theoretical Knudsen separation factors for gas/CO2 selectivities of various gas mixtures owing to extremely low CO2 permeance. The unique transport features of the low CO2 permeance were explained by the blocking effect of CO2 adsorbed in the nanoporous diffusion channels because of the high CO2 affinity of the edges of GO and rGO sheets. Furthermore, the rGO lamellar structure generally shows impermeable interlayer spacing, indicating that the only gas diffusion channel is the nanopores created by neighboring the edges of the rGO sheets. Notably, both membranes maintained a higher H2/CO2 separation factor than the theoretical Knudsen selectivity, including the measurements of mixed-gas permeation experiments. This study provides insight that further GO modification may improve the gas separation performance suitable for specific separation processes.