Time resolved photoluminescence (TRPL) is employed to study photogenerated electron-hole plasma expansion under strong illumination conditions in thin GaAs epilayer. We have observed spectral dependencies similar to ones which are usually observed on carrier's cooling. However, in our case they are caused by the diffusion of the degenerate electron-hole plasma perpendicular to the crystal surface. We have studied the influence of the degenerated carrier transport on the luminescence characteristics of bulk samples, both experimentally and by simulations. We suggest using this method to measure the transport coefficients of both degenerate and hot excess carriers in semiconductor structures.