User blog comment:FairyRaven/Fairy Tail Chapter 366: 1000 Souls, Review and Commentary/@comment-6904925-20140110110507

You know, I think its more of chance that Silver is Future Gray, instead of his father, because if Natsu can tell if people are related by smelling them, then couldn't he have notice that Cana and Gildarts were related, but he only found that out after Cana told Gildarts.

Or, after ForneverWorld gave his review on the chapter on Youtube (http://www.youtube.com/watch?v=P1w9o9u75so) he mentioned that Silver could be Gray's demon counterpart, and the demons are from their own world, like Edolas, where everyone have their own demon counterparts.