User blog comment:Miskos3/Fairy Tail 521: The Strongest Wizard, Review/@comment-29778526-20170211193404/@comment-29778526-20170213035644

Now that I think about maybe Gray trump card is a mode. I mean Dragon Slayers had different modes based on their elements, like Natsu Lightning-Flame dragon mode, Gajeel Iron-Shadow dragon mode along with future Rogue an Sting similar modes.

So maybe Gray had a devil mode of his own that he will use to take on Zeref, just as Natsu had when he used his Fire Dragon King mode to fight Zeref and he would have won, if Happy didn't interfered in the last attack.