User blog comment:Miskos3/Fairy Tail 530: Neo Eclipse, Review/@comment-7418318-20170408205108/@comment-31699381-20170408223008

Lucy actually explains that. She says it was due to the eclipse that was happening that night, the gate was acting a doorway to 400 years in the past. she mentions that if it was any other night, the user could select the point in time the want to go to.

I also find the purpose the the gate and the GMG to be quite paradoxical. Before future rogue hijacked it for his plans, they intended to use to go back in time to the point where Zeref wasn't immortal and kill him. But this would have been before he invented the gate leading to the gate never being invented.