Correction - he wasn't found guilty of breaking the law. We have limited information on what actually happened, and one interpretation of the data is that Zimmerman threatened or initiated aggression - effectively committing assault, and thus taking him out of "stand your ground" territory.
Be careful of apples and oranges: You are comparing your third-person-omniscient knowledge of what is happening with the hypothetical gang member with the real case of Zimmerman, for which we do not have that omniscient viewpoint.
So, step back your view on that gang member. Reduce the information you have about him, and the case, to make it equivalent. "Gang member" is going to be difficult to actually prove in court, unless you get one of his fellow gang members to rat on him, so that's really not an issue. Say the weapon itself gets excluded from evidence, on procedural grounds. Or, perhaps more typical for the expected issue, the gang member doesn't have any prior convictions, and, knowing he's in a SYG state, figures he can go ahead and use a gun he legally owns! He is carrying the weapon legally! He can't use it in any robbery, but in any drug-deal gone bad, or in any confrontation with another gang, he may be able to claim SYG if he legally owns a weapon.
"I'm sorry, Your Honor, but I was minding my own business, when this guy came up and started tellin' me to get the heck off 'his turf'. I was just goin' to the corner store for some smokes, Y'r Honor. It looked like he was goin' for a knife, so I shot him."