User blog comment:FairyRaven/Fairy Tail Chapter 331: Natsu's Strategy, Review and Commentary/@comment-3290324-20130517205151/@comment-7418318-20130518172011

So far we only know that non-Dragon Slayer magic can't pierce through them, but that doesn't mean they are not affected.

That Mira was able to punch Zirconis' mouth closed by hitting his lower jaw would support that even when the magic fails to penetrate the scales, the kinetic impact still remains, at least partially.

Also we don't knwo how exactly Gravity Change works for sure. If gravity magic changed the natural gravity around (as the name would suggest), rather than applying a new, purely magical force, it has good chances of affecting the Dragons (though with their immense strength the caster would probably need to feed a lot of power into the spell for it to work in any meaningfull way).