I always wondered about this. If changes in gravity propagated faster than light, could you build a transmitter by modulating a gravity field by colliding matter with anti-matter?
If you're thinking that the matter and antimatter have gravity and that gravity disappears when they annihilate because their mass is gone, that's not the case. All energy and momentum produces gravity, not just mass.
It's a fun exercise to calculate just how strong a laser beam you'd need to have to make a black hole entirely out of photons.