The Collaborative International Dictionary of English v.0.48:

Gnar \Gnar\, v. i. [imp. & p. p. Gnarred; p. pr. & vb. n.
   Gnarring.] [See Gnarl.]
   To gnarl; to snarl; to growl; -- written also gnarr.
   [Archaic]
   [1913 Webster]

         At them he gan to rear his bristles strong,
         And felly gnarre.                        --Spenser.
   [1913 Webster]

         A thousand wants
         Gnarr at the heels of men.               --Tennison.
   [1913 Webster]