My question is if I wake up the usual time for Sehri before Fajr, and then I realize that I need to do ghusl al-Jannaba, but instead I do a quick Sehri, and go to do the ghusl 4-5 mins before the Adhan of Fajr, and when I come out of the bathroom the Fajr Adhan has already take place, so is this fast valid?

If dawn started before finishing the ghusl, then you need to re make this day.