Liquid-gas two-phase flow in the gas diffusion layer (GDL) of proton exchange membrane fuel cells is investigated using an ex-situ experimental setup. The mass transport phenomena is investigated in carbon paper and carbon cloth GDLs and at different compressions. Water percolation within the plane of the GDL is visualized with a CCD camera while its injection pressure is measured. Similarly, air percolation within the plane of GDL samples which were initially saturated with water is investigated. Experiments are conducted for the three flow regimes of stable displacement, capillary fingering, and viscous fingering. Images are analyzed to obtain the normalized wetted area. It is observed that while the GDL compression directly affects normalized wetted area for stable displacement and viscous fingering flow regimes, it has no impact on this parameter for capillary fingering flow regime. For stable displacement flow regime and for both carbon paper and carbon cloth samples, water percolation pressure increases with GDL compression. However, the water percolation pressure data obtained for capillary fingering flow regime does not suggest any discernible trend as a function of GDL compression. The findings in this study can be used to validate percolation models proposed by different schemes such as pore-network models.