Take two objects like the Earth and the sun. Before light can reach the Earth from the sun the distance between them would need to be defined. Gravity is what defines this distance, right? So gravity would have to get here first before the light did. The only way this happens is that the speed of light is shifted less than what it is to the stationary Earth observer. However light is actually faster than this speed once Earth starts moving. If gravity was not faster than light how could it escape from a black hole?