Remember, he called the non emergency number. We don't know how long it would have taken them to arrive if there hadn't been 911 calls of a shooting. Maybe they were on the way and would have arrived the same time regardless, but I think it's fair to think that GZ was thinking it might be quite some time before they arrived.
It doesn't matter who he called, or how long it took to get there. It was beyond GZ's responsibilities to do more than observe. He's Neighborhood "Watch" not Neighborhood "Police."
He observed, he called, end of story...or at least it should have been.