What is the ruling on the fast of one who became in a state of major ritual impurity (junub) on a night in Ramadan and forgot to perform ghusl until an hour after Fajr prayer, then performed ghusl and prayed again?

What you did by performing the ghusl (ritual bath) and repeating the prayer is correct, because ritual purity (tahara) is a condition for the validity of prayer, due to Allah's saying: ﴿And if you are in a state of janabah, then purify yourselves﴾ [Al-Ma'idah:6]. As for waking up in a state of janabah, it does not affect the validity of the fast, even if one did so intentionally, as it is narrated in Muslim from Aisha, may Allah be pleased with her, who said: "The Prophet, peace and blessings be upon him, would wake up in a state of janabah not due to a wet dream, and then he would fast."