What proof do you have trayvon knew about the gun before assaulting Zimmerman?
There are a lot of reasons that could lead a person to believe the gun caused the fight.
The lengths Zimmerman went through to have Martin questioned, his physical direction, his recorded commentary at that time, the absolutely perfect way a gun lends itself toward accomplishing such a goal - without a single other consideration, could lead a person to believe that the gun caused the fight.
Had he chosen to hold Martin at safe gunpoint until police arrived, he would have ended up on his face, himself, until the situation was secured. He would have lost his permit to carry in the absence of an extremely good reason for his behavior, and would have likely suffered other consequences including a potential lawsuit from Martin.
So we have a situation where he really wants to have Martin questioned, and knows that the gun has potential to accomplish that for him; but if he's going to use it, he needs to do it in a way that doesn't involve drawing and pointing from a safe distance.