Schematic view of the collapsar at the onset of the GRB: stellar core (dark blue), stellar envelope composed of subsequent accreting shells with decreasing density (light blue-yellow-orange-green), and rotationally supported accretion disc formed at the equatorial region (red). The horizontal black line represents the equatorial plane. The circle marked with a stripped line represents an exemplary chosen radius above the horizon, at which the gas feels the perturbative force due to the self-gravity of the matter enclosed within this radius (see Eq. (7)).