If real (and I kinda think it's real, IMO), the "flat edge" of the blood hypothetically could mean falling on one's back, tapping one's head against the edge of a sidewalk during the altercation, or simply losing one's balance and falling backward.
A good chest push "let me go, get out of my way, man!" could have caused GZ a loss of balance, falling backward, if he was standing in front of TM, impeding TM's abilty to walk past/escape.
HYPOTHETICAL/JMO: GZ, is attempting to isolate TM until cops get there .. but doesn't ID himself or what authority he has (if any).
TZ zigs to get past GZ, GZ reaches instinctively for TM's sholder to slow him.
TM reacts to the shoulder grab, pushes back HARD, sending GZ backward... all this while in the grass, with the sidewalk behind GZ.
I can imagine GZ falling backward, smacking head. Again, utterly hypothetical, and JMO. this is just what I glean from the shape of the mark and blood.
However, I don't think that one linear mark looks like multiple bashings as described. I'd expect repeat dissimilar gashes/scrapes, if this "head bashing took place.". JMO