If gravity were an instantaneous force then gravity waves would not be possible. A wave is, by definition, a space/time event, a rarefaction/compression event of something, air, water, space time, that travels at a predictable speed. The cessation of an Instantaneous gravity field would occur as a state change, not a wave. That photons travel as wave forms suggests that they interact with space/time in a similar manner as does mass...both distort space/time and those distortions travel through space time at the same rate, just as the wake of a boat travels outward at a set speed, irrespective of the speed of the boat.