Here is a scenario that IMO agrees with the witnesses, evidence, timeline and police report as we have them at this point. Disclaimer -- This is MOO and speculation only:
A physical fight has begun. TM has the upper hand and is on top. GZ's head hits the sidewalk a couple of times. They are grappling violently on the ground. Both men are vocalizing loudly. GZ sees the bystander (witness) and calls out for him to help. Bystander yells at "him" (who?) "to stop" but keeps his distance from the fighting men. Says he is calling 911 and leaves the scene to do so. GZ and TM are now alone. Struggle continues and escalates to the tipping point. GZ shoots TM at close range, abruptly ending the fight. TM stands or staggers for a moment as GZ scrambles to his feet. TM, mortally wounded, crumples or falls to the ground face down. GZ then straddles TM (perhaps pinning him for his own safety just in case?) and checks TM for a weapon and/or signs of life, etc. Witnesses immediately begin to emerge/gather. LE arrives at the scene almost instantly, as a unit was already on its way due to GZ's earlier call to report a suspicious person.
Not saying I believe that is what happened, and I am not on anyone's "side." IMO, without more factual evidence to consider, none of us can know with any certainty in this climate of heated rhetoric, misinformation and hysteria. Just explaining one example of how TM could end up face down after being shot, with GZ standing over or straddling him in the moments after he went down. JMO