It wasn't "his ground" however, in this case!
If GZ never got out of his truck, knowing the police are in route (as he was instructed and also per the regs of his "neighborhood watch" group) a senseless death would have been avoided.
What law did GZ break by getting out of his truck?
The gal who trained him for the neighborhood watch program had the booklet that said not to follow suspicious people, you can't ask them what they're doing here, stuff like that.
She testified. He was even calling Trayvon a suspect.
Not to mention if he had stayed in the truck, the kid would be alive.