I thought your story was plausible until you got to the part about struggling for the gun. This was clearly a one-sided fight. If it happened the way you said, then TM would have injuries as well, he had none. The fight was so one-sided that had TM knew and struggled for the gun, he would have had no problem getting it away from GZ. What you described doesn't account for the severity of GZ's injuries as well as the almost pristine condition of TM (minus the bullet hole-he didn't even have dna under his nails suggesting a close-fisted fight, not open palm struggle)
No one throws punches or even goes for the face when a gun is involved. You struggle for it, and the winner then points it at the loser or throws it away. You don't hand out an ass-whooping (GZ was clearly on the losing end of a beat down) unless the gun is in your possession or no longer an issue.
The earliest TM could have known about the gun was mid-ass whoop.
PS. I also find the idea of TM hiding a bit of stretch (though arguable). The guy was on the phone with Jeantel. Who the hell hides, but continues to have a regular conversation on the phone?