Motbydlig is used when something disgustes you, "Hun er motbydlig", "She is disgusting". Ussel is used when a person is often weak in a disgusting way, like, paltry and petty. "Hun er et usselt menneske" "She is a petty human". Vemmelig is used to describe something nasty and sickening, "Maten var vemmelig", "The food was sickening". Gruelig, "Å grue seg" means "To dread", so gruelig is something like dreadful or horrible, a mix of those two words. Something that you were dreading. "Det var gruelig", "It was dreadful" Vederstyggelig, this word is never used, I had actually never heard it before I read this, so I had to look it up. It is an old word and means "To feel disgust (with)".