Gravity is known to affect heterogeneous combustion in porous media; it is especially appreciable at very low air (oxidizer) flow rates. Effect of gravity on the combustion of porous objects includes not only the force of gravity (gravity field inside the porous object), but also the pressure difference at boundaries of the porous object, which is caused by the action of gravity on the ambient air if the open borders of the object are located at different heights. In this work, it has been studied numerically how the gravity field inside the porous object and the above-mentioned pressure difference at the object boundaries affect the combustion process in the object. It is revealed that the gravity field inside the porous object and the pressure difference at object boundaries lead to opposite effects: the pressure difference on object borders leads to the propagation of the combustion wave in the same direction and the same mode as in the vertical porous object, but the gravity field leads to the spread of the wave in the opposite direction or in a different mode. In the vertical porous object the pressure difference at object boundaries defeats the effect of gravity field inside the porous object, but the gravity force leads to reducing the velocity of combustion wave.