From what I've read, a jury is supposed to receive instruction for manslaughter if the evidence would be consistent with such a verdict. From my understanding, that would essentially require finding that GZ pulled the trigger recklessly but without intending to shoot TM. I have seen no evidence by either side to support such a claim; if the defense were to stipulate that GZ was trying to aim at TM when he pulled the trigger, and state that it did not want the jury to be offered manslaughter as an option, I would see no basis on which Judge Nelson could a find a manslaughter verdict to be consistent with the evidence.
They say that Florida law does not allow for a sliding scale, the jury cannot reduce the verdict. It is the same in NY, where the jury let some guy go who shot 4 black men in the subway and then shot one of them again in the head. They couldn’t reduce the charges, so the declared him not guilty.