It could just as well be a green shirt with the name of a sports team written on it. Franklin has a shirt with LS and a crown written on it, which could mean that the gang is called 'Los Santos Kings', At least add notifications that your article is pure speculation.

The female also has a Magnetics t-shirt.Dodo8 Talk

It has an Ⓡ, registered trademark sign after the Magnetics name. It being a gang name is highly unlikely, since it is a registered trademark.