Board Thread:Character Discussion/@comment-34924658-20161007024033/@comment-25375217-20170417191626

Perhaps Gold is the surname he took after he became the Dark One? As I mentioned before, there would be no reason for Malcolm to have a surname, but once Rumple became the Dark One and everyone knew him, he may have wanted one... Ooh! Here's a good character motivation for it: he took the last name Gold around the same time he killed Fendrake - because he thought he was going to start a family with Cora and they fell in love over spinning straw into gold (and other magical things).

(we'll just ignore the pre-Dark One contract using Mr. Gold because that really just doesn't make sense. Even if he did have a surname, why would it be Gold?)