While I was surprised to learn that Gosetsu was a Roe (his facial features seem much closer to Hyur, so I assumed he was just an exceptionally burly Hyur), it may be too soon to postulate a new clan. The Live Letter says, "Rather than a Sea Wolf, he's closer to the Hellsguard tribe." This gives them plenty of space to simply say that Gosetsu and his people are immigrants/descended from the Hellsguard.