User blog comment:Miskos3/Fairy Tail 520: Dragon or Demon, Review/@comment-24102956-20170204101737/@comment-7418318-20170206110448

But Gajeel was still in the portal between worlds when Universe One shifted him... a portal that no longer exists.

Hmm... does that mean that his lower half will be pulled to the underworld, while his torso, head and arms stay in Fiore? :P

Joking aside, if he had already been pulled to the underworld, he wouldn't have been affected by Universe One, so we can savely assume that his point of origin was still in the world of the living and thus he'll stay in the world of the living without even having to retcon anything. And since the threat of death was already cured by the Tenrou Tree's blessing there's also no danger of him suddenly dropping dead.

He might have been saved by a massive coincidence against terrible odds, but at least the method in which he was saved itself made sense.