Long before Gakirah Barnes was murdered in a hail of bullets on the South Side, her mother had heard the unsettling, persistent rumors about her. Her 17-year-old daughter — the same 5-foot-3, 128 pound girl once seen as shy at school — was known for something almost unheard of on the streets of Chicago. There are teenage girls who are accomplices in gang murders. There are girls who hide gang guns for their boyfriends. But Barnes was believed to be a “hitta” — a female assassin for a faction of the Gangster Disciples, law enforcement sources say.