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I presume your bystander perceives the traveller to be moving away from the gravitating mass with constant velocity. That's a very complicated scenario. The thrust of the rocket engine has to exceed the force gravity for a while in order to achieve cruising speed. It then has to be reduced abruptly to match the force of gravity at the point where cruising speed is achieved and backed off gradually thereafter to maintain cruising speed as the force of gravity recedes. The rocket engine adds energy to the traveller at a variable rate and that alters the space-time continuum in an unknown manner. Furthermore, the rocket loses mass over time as it generates thrust.