If GZ had grabbed him physically of course it would be reasonable to protect himself and fight him back. We don't know what happened. HOWEVER, even if he did fight GZ from grabbing him, he can't use excessive force once the threat is over so continuing down the walkway grappling, mounting, punching banging head etc. would have been over the top. If GZ had pulled out his gun, jmo, but I think it is DOUBTFUL anyone would attack someone who is holding a gun. They would have had the "what's your problem" conversation. TM would have been shot prior to getting control over GZ.
Have any of you ever been in a fight? That might be a problem for me if this were my jury. You can't really understand the dynamics of what it feels like to take a hit when you aren't used to it. A punch in the face is very stunning. A punch from a man anywhere in the head or torso is stunning if you are not accustomed to taking hits or blows.
Also, the positions and body weight distributions change in a grappling situation. I watch a lot of MMA UFC and used to do martial arts. It's possible to be pinning someone down with your legs with their face exposed and the next second he's got a hand free, and then when you try to regain control, weight shifts and it changes the whole scenario. All the things talked about were possible...TM pinning GZ, TM sliding down or GZ sliding up, GZ suddenly being able to free an arm and reach for a gun especially if TM is reaching his right hand across toward the left.
Also, when you learn to fight, you start with jabs with your non-dominant hand, followed by a punch with the dominant hand - the 1,2 punch. Jabs can be as effective as hits. People who fight use both hands. Street fighters and trained fighters. If you are holding a cell phone in your right hand, you might punch a guy with the left.
The point is, with so many possibilities, there is reasonable doubt.
In MY OPINION ONLY