Following is not against the law. Putting your hands on someone is. According to the gf - she heard GZ say something to the effect "what are you doing here" - so GZ was right there with Trayvon. Again - common sense and logic tells me that regardless of what Trayvon said to him, he wasn't going to let him get away and therefore reached out and grabbed him by the arm. Trayvon now, felling threatened, punched GZ in the nose is an effort to try to get away - or at least get GZ's hand off him.
What we need to know - and probably never will - is who put whose hands on the other one FIRST. Whoever that was became the aggressor.
Again, common sense and logic tells me that Trayvon did not "touch" GZ first - we know he was afraid and had relayed to his gf that "some dude was following him" so why would he touch him? Elude and evade - that's why Trayvon was probably trying to do, yet GZ caught up to him and grabbed a hold of him.
JMHO (an opinion is all we will ever have of this - GZ will never say he touched Trayvon first and Trayvon can't speak for himself)