Full and approximate models describing changes in gas pressure for non-stationary single- and double-reservoir methods of testing of gas permeability of porous materials were formulated. The models include effects of compressibility and slip of gas. For derivation of the approximate models it was assumed that a transient mass flux of gas is constant along the tested sample (spatially homogeneous). The assumption allowed finding analytical solutions for evolution of reservoir pressures as well as the transient spatial distributions of pore pressure in tested samples. The full model was solved with help of the finite element method and the results were compared with the derived approximations. The comparison shows very good agreement of predictions of both models. Parametric studies have given the evaluation of the role of selected material parameters (permeability, porosity and slip factor) and parameters of the experimental system (initial pressure, volume of reservoirs), especially important from the point of view of identification procedures of the material parameters.