The gravitational waves here were measured for about 100 seconds (as opposed to the much shorter 'chirp' from the first gravitational wave detection made in 2016) so the whole event took some time to unfold.
The 2 second delay was due to the fact that gamma rays were generated by matter slowing down after it was ejected from the system and which collided with galactic gas. That matter was ejected from the black hole's axis of rotation and the black hole formed some time after the gravity waves were detected.
Take a look at this article: http://physicsworld.com/cws/article/news/2017/oct/16/spectac...
"As this material was sucked into the black hole, a fast-moving jet of material blasted outward along the black hole's axis of rotation. When this jet collided with gas in the galaxy, it started slowing down and the lost kinetic energy was broadcast as gamma rays"