Is using soap while performing ghusl after menstruation, out of ignorance that the Prophet (peace be upon him) only performed ghusl with water, considered a sin and does it necessitate anything?

Ghusl (ritual bath) from major ritual impurity (janabah) must be performed with pure water (al-ma' al-tahur). The use of soap or detergents is not a condition. If you cover your entire body with pure water, whether soap was used before or after, the ghusl is sufficient. The belief that soap is a condition is an invalid belief.