User blog comment:CNBA3/Fairy Tail 411 Review/@comment-1359322-20141201102857/@comment-9595671-20141201105755

I always figured that Dragon Slayers (minus the 2nd Gen) could learn how to access Dragon Force without outside aid, when Natsu said that Sting's Dragon Force was "half-baked" and I recalled the Lacrima that makes him a 3rd Gen slayer the message I got was: "Because my lacrima lets me use it freely, my Dragon Force was unable to fully develop like if I had just learned how to do it through training"

Well Gray is a Devil Slayer now, so maybe its something like a Demon/Devil Force (though its hard to say considering that we haven't seen a God Force used by God Slayers)