User blog comment:IamJakuhoRaikoben/Chapter 411: Quid Pro Quo, Review./@comment-4449950-20141201232911/@comment-7418318-20141202005637

I have the theory that maybe Gray did what slayers do (or at least Natsu and Gajeel did): He ate the Memento Mori curse (Devil Slayer should be able to eat curses right?).

For some reason or the other though, Gray couldn't fully eat it and still took quite some damage, but still managed to gain some magic/curse power from it to put into his final attack.

Now why couldn't he absorb it fully? For this I have three theories:
 * 1) He's a noob who just learned the magic and thus wasn't good at it. Natsu was only able to absorb and fully integrate Laxus lightning at the 2nd try after all.
 * 2) He's not a real Devil Slayer, but just got the power temporarily and isn't able to fully utilize it.
 * 3) The spell was just too strong for him to fully take in, kinda like Natsu was in extreme pain and choking when he ate Etherion.

Hopefully a combination of all three. :P