I have listened extensively to all scream recordings, both in the context of full 911 calls as well as isolated screams cut and put into one file.
I have listened to GZ's voice extensively, and to TM's a small amount (as much as is currently available) TM had a substantially deeper voice, as his father Tracy Martin does.
I am certain it was GZ screaming, based on his injuries, his account, John's account, logic of whether someone sustaining those injuries would scream like that (the screams are of absolute terror and pain) whether someone with a broken nose which is being hit again would scream, and whether someone administering a beating would scream (they wouldn't, or if they did it would probably be more intelligible and of the nature of "this guy's got a gun! someone help me hold him down!" something...)
I don't think there is any sort of way to believe it was TM other than via ignorance, emotion, or both.
Sorry to be so harsh but, it is clearly GZ's voice too.
I also happen to think it is ONLY GZ screaming and that, at least on the recordings we have, TM never screamed.