그만 means 'STOP' 그만해. 그만! 만큼 is different to 그만

만큼 means "as much," or "as much as~." i.e. 사람들은 돈을 번만큼 써요: People spend as much money as they make. 이만큼: this much 저만큼/그만큼: that much 나는 이만큼만 먹을께요: I will eat only this much. 그만 is usually followed by verbs. "그만 + verb" means "to cease to + verb." i.e. 그만 해!: Stop doing it = Stop it! (or, you can simply say, "그만!") 그만 먹어: Stop eating 그만 자고 일어나세요: Please stop sleeping and get up. Hope this helps.