User blog comment:FairyRaven/Fairy Tail Chapter 398: The Final Duels, Review and Commentary/@comment-46.190.6.2-20140901102606/@comment-7418318-20140902095134

Tempesta did start to look surprised and worried the moment he saw Gray's weird eyes.

I interprete this as Tempesta knowing that Gray somehow got Devil Slaying, but didn't expect him to be quite this powerful and skillful with it until his storms got frozen and by then it was already too late.