Diffusion of solvent gas in heavy oil and bitumen is essential in solvent-based heavy oil recovery. The pressure decay technique used for prediction of gas diffusion coefficient in heavy oil ignores the role of changes in gas compressibility factor and oil swelling. In this work, a numerical approach is proposed to model gas diffusion in bitumen from pressure decay data which includes all complexities due to oil swelling and gas compressibility factor. The proposed model was solved using differential quadrature element-incremental method. Results showed that changes in gas compressibility factor and swelling factor are significant and cannot be overlooked while interpreting pressure decay data. For diffusion of gas in bitumen, swelling was more significant at upper sub-layers adjacent to the gas phase. Also, swelling is more pronounced in the upper bitumen layers in contact with gas and stabilizes at later times when the gas concentration reaches equilibrium at the interface.