It is my understanding that things accelerate towards centers of gravity, but once an object reaches the bottom of a 'gravity well' its speed will no longer grow (in fact the object will lose speed as it tries to leave the gravitational field). In order to accelerate something to the speed of light, you would probably need a field of infinite gravity, therefore you would need infinite mass, and infinite mass means that it would take infinite energy. All of this means that you cannot use gravity to accelerate an object to c for the same reason that you cannot accelerate a space craft to c using energy; you don't have access to an unlimited amount of energy.