The little I've read about the spinoff is that it's five tribes of... werewolves? All werewolves? Something makes me think that some of them are vampires, but I'm sure not, considering they're probably going for the Teen Wolf groupies. Based in Chicago. And there is a hunter who is trying to pick them all off. Presumably some tribes are good and some are evil and some are just whatever, we don't believe in the good/evil dichotomy.

I don't think Garth has anything to do with it, as DJ has another series (something popular, I think) and it was near impossible getting him back on Supernatural at all.