All of these words are adjectives (not adverbs, despite the -ly ending).
Grisly means “causing a feeling of horror or disgust,” such as images or stories of violence and disease. The root of the word—gris—originally meant fear.
Gristly describes gristle, or rather meat containing gristle; gristle is the hard, stringy, chewy material, made mostly of cartilage, found in some pieces of meat, which is considered undesirable.
Grizzled describes hair, often facial hair, containing a mixture of gray and dark hairs. The word is most often used to describe older men with a “rough” look—men who look as if they have been to war, for example.
Grizzly—not only an adjective, but also a noun—is short for grizzly bear, a species of bear which has gray mixed in with its brown fur. Grizzlies are large bears, common in the North American wilderness, especially in the Rocky Mountains.