[wlesieutre]

https://arxiv.org/abs/1710.05833

> On 2017 August 17 a binary neutron star coalescence candidate (later designated GW170817) with merger time 12:41:04 UTC was observed through gravitational waves by the Advanced LIGO and Advanced Virgo detectors. The Fermi Gamma-ray Burst Monitor independently detected a gamma-ray burst (GRB 170817A) with a time delay of ∼1.7 s with respect to the merger time.

So there is a delay between arrival of gravitational waves and accompanying gamma ray burst, but I couldn't tell you if that's purely because light travels slower than it would in a perfect vacuum, because the gravitational waves are generated before the gamma ray burst, or a bit of both. The GRB being less than two seconds long, I would guess they both happened at close to the same time, and it does have a speed difference.

Coming from an object 130 million light years away, 1.7 seconds is a very small difference in speed.