In fuel cells, a homogeneous distribution of gas flow is desirable for optimal performance. The gas diffusion layer (GDL) often used in PEM-like fuel cells is one of the key elements responsible for a uniform distribution under channels and ribs. To assess this ability of GDL-materials, characteristic numbers, e.g. the permeability, are often introduced. In this paper, we simulate one and two component gas flow through a virtual GDL material under operating conditions of a HT-PEFC. We observe the influence of discretization and viscosity choice on the macroscopic output of the Lattice-Boltzmann algorithm we apply. To achieve this, we first study their effects on empty square channels and finally transfer our interpretation to the output of a GDL simulation.