I don't think GZ ever had the right to defend himself with lethal force because he was never in danger of great bodily harm.
And if the jury agrees there was no reasonable fear, beyond any reasonable doubt, he'll be convicted of manslaughter. But it is a lot easier to make such calls in hindsight as a passive observer, particularly if you've graduated from the school for internet tuff guys. And just out of curiosity, what gives you any certainty at all that TM would have stopped beating GZ before the injuries got much worse?
To determine reasonable fear, you must put yourself in GZ's shoes, pinned under an aggressor, taking hits to the head.
If this jury does that, then coupled with the bloody photos, I see no chance of conviction.