As neutron stars are small, like 20 kilometers, while the light travels 600 000 km within 2 seconds and one can roughly assume that speed of processes that generates the gamma burst during the stars collisions matches the speed of light the delay may need some explanations. It could be just scattering of gamma rays by intergalactic medium that does not affect the gravitational waves, but what ever it is, I suppose it matches models or this delay will be in the news.