Sett is very far from being a top laner of level S. Already before 10.8 it was difficult that he had good matchups because of his lack of engages, after the nerf of the W I would put him safely in the tiers below.
I think Yorick should be a lot higher than D tier, as he has a 53% WR in all ranks. Although he fares poorly vs. picks like Tryndamere, Jax, Maokai, and Wukong, I still think that Yorick is a simple to play champion who can reward you greatly. Not only that, but he can beat Darius easily in lane, as long as you play passive till' 3. Not only that, but his Q has a 140% AD ratio. So... Yorick, although not in the meta necessarily, is still a really good top lane champion. Definitely still viable.
Personally don't think Gnar should be A tier. He transforms in team battles and when you are simply trying to poke. I personally think that Mundo is better than Gnar
I've played him today but not really the months before so I can't say for sure but I think he deserves a B at least.