Gnaw off is a phrase that denotes a specific action, which refers to biting or chewing something until it breaks off. Generally, it is used to describe the act of animals, that use their teeth to break off parts of plants or prey. However, there are many similar words that you can use to describe this action, including nip, chew, bite, tear, and masticate. Each of these words depicts the concept of breaking off something using teeth and jaws. For example, you could say that the beaver was nibbling away at the tree trunk or that the lion was tearing off chunks of flesh from its prey. Ultimately, choosing the right synonym depends on the context and the effect you want to create in your writing.