The paper is devoted to a theoretical analysis of a counter-current gas-liquid flow between two inclined plates. We used the Navier-Stokes equations for both liquid and gas to compute the nonlinear wavy regimes of the flow over a wide variation of the liquid Reynolds number and the gas superficial velocity. The main interest is to analyze the variation of the free surface shape, u-velocities and phase velocity of the waves as the superficial gas velocity is increased. We do our investigation for relatively small values of the distance between the plates where the shear modes of the linear disturbances are stable and the gas flow is laminar. We found that with the superficial gas velocity increasing and starting from some critical value of this velocity, the dependences of the main characteristics of the waves demonstrate a qualitative modification. For example, at small values of the plate's inclination angle the dependences of the gas friction coefficient and the averaged film thickness have a local minimum for different values of the liquid Reynolds number at this critical value of the gas superficial velocity. We observe an essential increasing of the portion of the wavy profile with negative values of the u-velocity on the interface starting with this critical velocity. The obtained critical velocities are in reasonable agreement with the onset of flooding observed in experiments and their values have a correct trend with the increasing of the liquid Reynolds number. (C) 2018 Elsevier Ltd. All rights reserved.