Well he still is a Gennin because with everything going on he have never have a chance to take the Exam... However, becoming a Hokage does not require you to have those rank. Also everyone in the village know he on a totally different rank then any of them can hope to achieve. Sage mode is basically put him in the class of Legendary Ninja like the Fifth Hokage, Ochimaru, and the pervy sage.
Also if you notice the rank only mean what kind of mission is available to them. Considering how many Atasuki he have taken out by himself including the man who stole the heart of the first Hokage what difference does it make that they call him a gennin?