I've never seen this graffiti before. From what I can see, I see the Star of David in there, so I'm assuming that it must be related to Judaism, the pessimist inside thinks it might be anti-semitic. I also see a 4 in there, but it might be one of those fork shapes above the Star of David unfinished. I also see some sort of diamond shaped G inside of the Star of David. So my best guess is that this is some gang with a name that begins with G that is anti-semitic, and this is their symbol. But that's just the best thing I can come up with after just seeing this graffiti for the first time 5 minutes ago. Who knows.
It looks like the symbol that those people were using in the walking dead video game when they were in atlanta to mark what houses were searched, infected, or safe so on and so forth. Iirc they used something very similar. That said I don't believe it was the Star of David in the game.