@Jeruba – I believe that’s the theory, assuming you could instantly add or subtract mass from something, the change in it’s gravitational force would need to travel.
I had a hard time wrapping my head around it until I thought of it in other terms. If you have a electromagnet and flip the switch to pass electricity through it, you’re essentially upping the force exerted by the magnet and it would make sense that it may take a moment for that force to reach anything that would be affected by it.
So gravity travels just like a shock wave, light, sound, or any other force. We just perceive it to be constant because the only massive object that really effects us in a noticeable way (the Earth) is constant.