User blog comment:IamJakuhoRaikoben/Chapter 487: The Third Seal, Review./@comment-24800928-20160601044110/@comment-7418318-20160601133204

As we've seen in the Tenrou arc, Gajeel can eat the iron that Levy "writes", so he'd get energy from it like when he eats any other iron, so it would certainly help... whether that by itself is enough to get him (who's never been in DF before) into Dragon Force is a different question though.

But it certainly would make more sense for him to go into DF by eating his element rather than something that destroys magic and just happens to have trace-amounts of iron in it.