What is the ruling on the initial ghusl (ritual bath), and is it obligatory to repeat all missed prayers after discovering a small amount of dirt attached to the body during the ghusl?

If the dirt is congealed sweat, it does not harm nor prevent the validity of ghusl. If it is a [product of] waswasah (whispers/obsessive thoughts), then pay no attention to it. If it is a small amount of dirt from outside the body, then you may act upon the opinion of those who overlook a slight barrier.