User blog comment:IamJakuhoRaikoben/Chapter 394: Juvia vs. Keith, Review./@comment-24800928-20140725181935/@comment-24066070-20140726091752

What I meant by that is that the power goes from parent to child.

Gray also had to spend years to "understand" ice and cold, so he already mastered the Ice-part. Since he channeled the Devil Slayer-part on 2 occasions, I'd say he mastered that as well. He managed to do so because of his already aqcuired mastery of Ice, which helped him extract the Devil Slayer powers from the Ice.

Gray's power boost isn't nearly as ludicrous as other power-ups have been in the past, like Second Origin or Natsu gaining Lightning (which isn't even his own freaking element)