No, you're not finding him guilty of murder; but it satisfies one of the elements of murder. Another element of murder is that the killing must be unlawful. Self-defence justifications make killings lawful; that is why he couldn't be convicted of murder, even if intent is established. As aforesaid, because he intended to shoot (hence kill) Martin, involuntary manslaughter wouldn't succeed.
You're incorrect about the other part mate: self-defence laws can give you a full defence to murder - of course that is if you can establish it (proportionality, reasonableness, etc).
Defence's argument wasn't that he didn't 'intend' to kill him but that he was using self-defence. In the legal sense "intend" does not necessarily mean he wanted to kill Martin in cold blood, but that his actions were intended to kill or cause grievous bodily harm. For example; if I were at your house and wanted to kill you but missed and killed your wife who was behind you; it doesn't matter that I didn't intend to kill her but that I intended to kill at all. I can still be found guilty of Murder. Manslaughter would be more like a situation where we both went hunting and I shot at what I thought was deer in the bush, which in fact happened to be you, and you died.
I don't know where you're going with this dude, invoking self-defence automatically means no intent!
And the other element of murder under Florida law is not that killing has to be unlawful - as long as you can prove "premeditation to kill", it is considered murder. It can still be murder if there's no pre-meditation, but someone dies while you're engaged in a pre-set list of illegal acts (like arson, trafficking, etc.). In other words, you don't need to show that the killing was pre-meditated and occurred as a result of an unlawful act - only one of those conditions would satisfy the requirements for murder under Florida law.
For manslaughter, you don't need to show pre-meditation, but you do need to show there was intent to kill, even if there was a provocation. The reason Zimmerman was acquitted was that invoking the right of self-defence and proving it, automatically meant that there was no intent, so that he could not be convicted of manslaughter.
Having said that the right of self-defence doesn't give you a blanket right to kill anyone and everyone. It simply gives you the right and justification to use force in defending yourself. That force must only be used to dispel the immediate threat that invokes self-defence and must be reasonable. In this case, it could have been argued that the use of force by GZ was NOT proportionate or reasonable in dispelling the immediate threat and therefore GZ acted with culpable negligence or recklessness, even though this homicide was not pre-meditated (murder), and he did not intend to kill Martin (manslaughter).
At least, that's the way I've understood Florida law.