The Collaborative International Dictionary of English v.0.48:

Gnarl \Gnarl\, v. i. [imp. & p. p. Gnarled; p. pr. & vb. n.
   Gnarling.] [From older gnar, prob. of imitative origin; cf.
   G. knarren, knurren. D. knorren, Sw. knorra, Dan. knurre.]
   To growl; to snarl.
   [1913 Webster]

         And wolves are gnarling who shall gnaw thee first.
                                                  --Shak.
   [1913 Webster]