There are a lot of different ways to go with this. And while Gabi appears guilty as sin, it could be she fired at Nick, or had the gun, intending to do so, but either her shots missed the mark, or she never fired but saw Nick go down. I am thinking that Lucas went to the park, but Nick was already down on the ground, Lucas left. He might even have had a gun with him as well.
The hitman headed for the park when he saw Julie had left. I also think he was too late.
There were people in the square together all the time.....Will & Sonny, Sami & EJ. Jordan/Allie. Actually, Abby and Ben, who were walking, saw each other. Rafe was there already. Kate arrived late as did Gabi, both joining Will & Sonny. Julie was last, and alone when she sat down, remarking that the place had filled up.
And yes, the person who did Nick in could very well come out of left field. Stefano or victor with a hitman, Brady, and definitely Percy....even someone HIRED by Percy.