Hawzah News Agency – If after performing ghusl one realizes that he has not washed an area of his body, in the event that the unwashed area happens to be on the left side of his body, it is sufficient for him to wash only the unwashed area. If the unwashed area happens to be on the right side of one’s body, the recommended precaution is that after washing the unwashed area, he should wash the entire left side again. Furthermore, if the unwashed area happens to be on one’s head and neck, then based on obligatory precaution he must wash his body again after he has washed the unwashed area.

Source: https://www.sistani.org/english/book/48/