The Collaborative International Dictionary of English v.0.48:

Gatch \Gatch\ (g[a^]ch), n. [Per. gach mortar.]
   Plaster as used in Persian architecture and decorative art.

   Gatch decoration, decoration in plaster often producing
      design of great beauty.

   Gatch work, work in which gatch is employed; also, articles
      of gatch ornamentation collectively.
      [Webster 1913 Suppl.]