I am going to assume that there is no solid proof. IF there was, they would have never signed him to that big contract a couple of years back. ALL OF A SUDDEN, after playing all these years in the NFL and NOW there are gang ties? Not 2 years ago? IF as a kid, you grew up in a crappy neighborhood with gang related issues...and you think none of the players in the NFL are not going to know KNOWN gang affiliated kids/young men?
This smells of the NEW head coaching not liking him and the brass NOT wanting to pay him the money.
Proof of anything isn't needed in the NFL. As was said earlier, the Eagles did not give a reason in their short statement last week. It may be as simple as Kelly not liking him, or it may be the gang stuff. We may never know.
What we do know is that Jackson won't suffer from this too much. He'll end up signing a comparable deal with another team, unless it comes out that all the gang rumors were right...