If your Ghusl water reached to every part of your outer body, then you don't need to repeat the Ghusl. You just need to wash the place which has a Najis item. But if the sperm covered a part of your skin and did not allow water of Ghusl to reach the skin beneath it , then your Ghusl was invalid because of not reaching to all your skin, and you need to re do the Ghusl again.
Wassalam.