If that is a bruise on the back of GZ's head it looks the same shape as the flashlight. GZ did say that TM jumped him and when he fell TM was banging his head on the sidewalk at the cut through. We know that is not where they ended up. So maybe GZ did fall at that point. Could be he started towards TM and dropped his flashlight and keys (which were found in that area) and slipped and fell backwards hitting his head on the flashlight. Or TM resisted GZ from stopping him. The shape of the bruise looks more likely he hit his head on it while it was on the ground rather than someone hitting him over the head with it which would produce a different shaped bruise, IMI.
Or GZ dropped his flashlight and TM saw an opportunity to jump him, GZ falls backward and lands on the flashlight?