All God/ dragon slayers have their attacks named "XXX God's (or dragon's) XXX", i.e. Zancrow's "Flame God's Bellow", Natsu's "Fire Dragon's Roar" and Chelia's "Sky God's Boreas".
But by far NONE of Orga's magic was named that way. Anyway there is also one exception in dragon slayers - Laxus. But as we all know, Laxus is a second generation dragon slayer.
Thus, my opinion is, Orga is probably NOT a God slayer - at least not a first generation God slayer.
Anyway, just speculation.