Gajeel was able to solo a Spriggan, but his fight ended up in a draw and therefore he would've died, if it weren't for Universe One. The reason why I ranked Erza higher is based, like I said, on overall character performance; in short, Erza was able to take down a Spriggan, with a little help from Bisca, and survive two more encounter with other Spriggans.
Gray has done nothing this arc to even rank him above Erza, so it would be absolutely ridiculous to do so.
Mirajane was ranked below Erza because she only faced Irene's subordinates and was exhausted at the end of the fight, but he power-up was pretty impressive and considering her opponents were Irene's creations, I ranked her below Gajeel.
As for Wendy being above Gray, there are two reasons: Wendy
blocked Dimaria's full-power attack in her base form, while her opponent was in her strongest form. And again, Gray hasn't confronted a Spriggan yet, so ranking him above these would be nonsense. The only wizard I ranked him above is Lucy.