User blog comment:IamJakuhoRaikoben/Chapter 431 & 432: My sword is... & Brair in Love, Review./@comment-26287497-20150429010954/@comment-9595671-20150429121043

I'm not sure how many have noticed, but when Natsu used his DF in the Tartarus arc without anything to help it activate Igneel wasn't inside of him. When Wendy used it Grandeeney hadn't come out yet, so its my theory that the Dragons were suppressing the ability.

They confirmed that Zeref wasn't why Acnologia became a dragon, so I'm assuming Dragon Force relates to it and they needed those anti-bodies to keep them from becoming dragons. I bet that when Natsu said Sting and Rogue's DFs were half-baked he was right, the Lacrimas may have let them used it freely but I bet Weisslogia and Skiadrum were still suppressing it making their DFs weaker than normal.

So its possible that Gajeel can use DF without an outside power to push him into it as well. Though I'm not sure if he can because Wendy and Natsu, both 1st Gen slayers transformed once before they used it freely, so Gajeel might need to do that first.