Some posters here have already mentioned the multitude of blood vessels in the head. Over a year ago, my sister fell backwards while going up some steps and hit her head on the sidewalk. After trying unsuccessfully to stop the bleeding for at least an hour, she finally called her bf, who took her to ER where she got stitches and a CT scan (risk of bleeding/blot clot in brain) and was admitted overnight for observation. Once, I suffered a blow to the nose (it was broken) and bled profusely. I was taken to urgent care, but they couldn't stop the bleeding so sent me to ER. When I later went to pick up a prescription (antibiotic) at the pharmacy, I realized that blood was splattered all over my clothes.
I don't doubt that GZ suffered some injuries from his scuffle with TM, and I know that the amount of bleeding depends on the thickness of the blood, but it is fact that it takes more than a few minutes to stop the bleeding from a serious head wound [there was also a time when my sister and I fell backwards off a porch swing onto a concrete porch when we were little kids. We both had to be taken to ER to stop the bleeding and get stitches - my sister on her head and me on my lip.]
TM was carrying a can of iced tea, a bag of Skittles, and a cell phone when GZ approached him. Maybe he put the bag of Skittles in his pocket, but he was talking on the phone at the time and had a can of iced tea, I presume, in the other hand. I remember reading that these items were found next him on the ground. So, I have a hard time believing that TM threw these items on the ground before pouncing on GZ as he was returning to his vehicle, spinning him around and punching him in the face.
I find the SYG law especially disturbing because most people (on other websites) who cite it as a defense in this case (and other cases, for that matter) suggest that the person carrying a gun is always justified. I.e., the idea that even if the person carrying the gun is the initiator of aggression, he/she should be absolved of guilt if the victim fights back. By the same logic, a rapist who attacks a woman as she walks down the street is justified in shooting her if she tries to fight him off. We don't know that GZ wasn't the aggressor in this case. If the testimony of TM's gf is correct, TM did attempt to avoid a confrontation with GZ. It was GZ who pursued TM.