"This is likely utterly naive, but if I understand the "communication" of gravity, it is instantaneous whether objects are juxtaposed or separated by a universe."
Actually this is incorrect. The effect of gravity actually moves at the speed of light according to Einstein's general theory of relativity.
As the article described, the force of gravity causes these stars to orbit around the black hole at the center of the galaxy. The size of the orbit depends on the velocity of the orbiting star (the slower the velocity, the smaller the orbit).
Ah, I didn't see your reply when composing mine.....