Gray wins this one imo. Gray is at least on par with DF Gajeel, if not, more. And DF Gajeel already has Erza outclassed by a large margin.
Contrary to popular opinion, I rank Gray among the top fighters for Ishgar (meaning on the level of Laxus and Natsu).
Gray demolished a Spriggan in base, and has yet to show his true power. Hell, he hasn't even shown us his trump card that has been hyped since the beginning of the war.
Regardless, Erza hasn't shown the same feats as Gray. Even if we say she won against Ajeel, it wasn't by much. Certainly not the same as utterly destroying Spriggan in base form.
So I'm voting Gray here. I'll put it at maybe mid-high diff max.
However, knowing the way this manga is written, Erza will probably beat Irene or something, which would mess up the powerranking quite a bit, but it's not like that hasn't happened before.