The arrival of the photons and the gravitational effects (gravity waves) attending the event arrive/occur in step.
Well, according to one theory of gravity. But i'm not sure we really know for certain, just yet, re: the wave nature OR "speed" of gravitational propagation.
One piece i read a while ago stated that gravity, in all meaningful calculations, is assumed to have instantaneous effects... and when they try dialing in a lightspeed factor for gravitational effects, nothing "works" as it's s'posed to, according to the reality measured. (i'm no doubt butchering the guy's presentation, here... but this was his general idea )
We know what gravity does, and we measure its effects (that we know of, anyhoo) down to the "n"th degree... but we still don't know what it actually IS.