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]

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]

WordNet (r) 3.0 (2006):

gnash
    v 1: grind together, of teeth