To gnarl is to twist something up until it's deformed. A gnarl is something twisted up like a knot.
When you gnarl something, you twist and mangle it. If you have long hair that's gnarled, it's become knotted and clumped up. You could say one of those clumps of hair is a gnarl. People's bodies can gnarl too, especially as they get older. An old person with crooked, poor posture has become gnarled. A tree that is twisted is gnarled. When something's gnarled, it's hard to untwist.