<p>Proton exchange membrane fuel cell (PEMFC) is a key technology to improve the utilization of renewable energy. To improve the performance and durability of PEMFCs, it is vital to enhance the gas mass transport capacity. To reveal the gas transport characteristics inside gas diffusion layers (GDL) of the PEMFC with gradient porosity distribution under different wetting conditions, the pore-scale model of coupled two-phase flow and gas transport processes based on the lattice Boltzmann method is developed to evaluate the performance of GDL under different gradient porosity distributions comprehensively. It is found that both the effective diffusion coefficient and permeability within GDLs with gradient porosity are affected by pore distribution, pore gradient, and liquid water saturation. With the increase of water saturation, the permeability in GDL with medium gradient porosity is relatively high. When the water saturation is 0.28, compared with the GDL under dry state, the permeability of GDLs with two-step, three-step, and linear porosity distributions decreased by 55.77%, 60.73%, and 58.69%, respectively. Besides, the flow resistance in GDL under linear porosity distribution is the lowest, followed by the one under two-step porosity distribution, and finally the one under three-step porosity distribution. Moreover, compared with the step gradient porosity distribution, the linear porosity distribution can effectively limit the droplet size and is less affected by the liquid water saturation. The linear structure under a medium porosity gradient can effectively improve the gas-liquid transport process inside the GDL.</p>