A big piece of this puzzle is going to depend on the forensics of that gun. IF both TM's and GZ's prints are on it - well, that clearly shows a struggle over the gun. If not and only GZ's prints - then, there is a problem.
The only way I see this being a self-defense case is what I outlined above. What I just don't understand is why GZ re-holstered the gun. That could have caused critical evidence - possibly in GZ's defense to maybe be wiped off. And if this was a close range shot - there is a good possibility that there was blood on it. Re-holstering it? IDK.
That's one of the big things, in my mind, that I'm waiting to see what the lab says. If TM's prints are not found on that gun - well, I don't see a self defense case holding any water.
JMHO
If there are two sets of prints on that gun and specifically, if TM's prints are on or near the gun handle, you're going to hard time prosecuting GZ IMO.