Gas purge intended to minimize residual water in the membrane, porous electrodes, and gas diffusion layers is critical for successful startup of a polymer electrolyte fuel cell (PEFC) from subfreezing temperatures. A basic understanding of the physical phenomena involved in gas purge, required to establish effective and energysaving purge protocols, is currently absent in the literature. In this work we delineate underlying physics of water removal from a PEFC during gas purge. Various stages of GDL and membrane drying are characterized and the variations along the flow direction are considered and emphasized. The predicted GDL drying time constant is compared with the tomographic experiments with good agreement. Detailed experimental validations are carried out and good match with experimental membrane HFR variation with purge time is obtained.