The only witness besides George to the start of the altercation was the girl on the phone with Trayvon. She testified that the last thing she heard was Trayvon saying "get off, get off". George has not witnessed Trayvon committing any crime. He does not have the right under Florida law to forcibly detain Trayvon. Doing so is considered assault.
Unfortunately her testimony is tainted by the failures of the police. This is one of the gravest of the injustices following the murder. In particular, she was interviewed late in the investigation process and in the presence of TM's mother. However the effects of the tainting are left to the jury to decide on.
What evidence led you to conclude that GZ "forcibly detain"ed TM?
From everything I have read or seen, the rule of law vindicated GZ. But justice was not served.