Great Sultan is one of various informal titles, such as Grand Turk, used to refer to the Ottoman Sultan, known in Ottoman Turkish as Padishah, Hünkar or Hakan, the sovereign of the Ottoman dynasty.
More appropriate would be to use the style Sultan, or rather Sultan of Sultans (Sultan us-Selatin in Ottoman Turkish or Sultan es-Salatin in Arabic), one of his many official titles.
Like Great Khan (but there the usage is so frequent since centuries that it can be considered an established convention) instead of Khagan, such translations do not render the subtle complexities of the original phrases.
A curious circumstance is that the Ottoman dynastic tradition was to give the title of sultan (elsewhere a Muslim ruler) to princes and princesses, merely as close relatives of the ruling Padishah, without an appanage (as the empire was indivisible).