User blog comment:Woest/Celestial Spirit King and Zeref/@comment-7418318-20131215093102/@comment-24066070-20131215190429

Since the royal family aren't even confirmed mages, I figured they would not be able to alter such a big project. They would just take the blueprint or whatever is in the book and replicate that.

With Zeref knowing everything about the Gate, I meant he may be able to operate it without the need to physically posses the golden keys. I didn't mean that he automatically senses the Gate whenever someone builds one. Even if he did sense it, there's a chance he also realized F.Rogue would pull some dragons through the Gate. Then he wouldn't have to start killing humanity, since he can just leave it to the dragons.