The gas is accelerated due to the introduction of energy into the system. Momentum is conserved because initially it accelerates in all directions. The shape of the chamber then changes the direction of the flow of the air, causing a momentum change. This is reacted by a force which pushes the engine, in accordance with the second law. In all practical terms, the change in momentum of the air has generated a force. The gas impinges on the turbine blades, reduces in velocity and hence momentum, and so once again a change in momentum has generated a force.
I'm sorry, but I still maintain that force and momentum change are inseparable, instantaneous, each indicates the presence of the other.