The Collaborative International Dictionary of English v.0.48:

Gast \Gast\ (g[.a]st), v. t. [OE. gasten, g[=ae]sten to
   frighten, akin to Goth. usgaisjan. See Aghast, Ghastly,
   and cf. Gaze.]
   To make aghast; to frighten; to terrify. See Aghast. [Obs.]
   --Chaucer. Shak.
   [1913 Webster]