The Collaborative International Dictionary of English v.0.48:

Gnash \Gnash\, v. t. [imp. & p. p. Gnashed; p. pr. & vb. n.
   Gnashing.] [OE. gnasten, gnaisten, cf. Icel. gnastan a
   gnashing, gn?sta to gnash, Dan.knaske, Sw. gnissla, D.
   knarsen, G. knirschen.]
   To strike together, as in anger or pain; as, to gnash the
   teeth.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Gnash \Gnash\, v. i.
   To grind or strike the teeth together.
   [1913 Webster]

         There they him laid,
         Gnashing for anguish, and despite, and shame. --Milton.
   [1913 Webster]

Easton's 1897 Bible Dictionary:

Gnash
Heb. harak, meaning "to grate the teeth", (Job 16:9; Ps. 112:10;
Lam. 2:16), denotes rage or sorrow. (See also Acts 7:54; Mark
9:18.)

Moby Thesaurus II by Grady Ward, 1.0:

25 Moby Thesaurus words for "gnash":
   bite, bolus, champ, chaw, chew, chew the cud, chew up, chomp, cud,
   gnaw, gob, grind, gum, masticate, morsel, mouth, mouthful, mumble,
   munch, nibble, nip, quid, ruminate, snap, swallow