The Collaborative International Dictionary of English v.0.48:

Gatepost \Gate"post`\ (g[=a]t"p[=o]st`), n.
   1. A post to which a gate is hung; -- called also swinging
      post or hinging post.
      [1913 Webster]

   2. A post against which a gate closes; -- called also
      shutting post.
      [1913 Webster]

WordNet (r) 3.0 (2006):

gatepost
    n 1: either of two posts that bound a gate

Moby Thesaurus II by Grady Ward, 1.0:

49 Moby Thesaurus words for "gatepost":
   French door, Samson post, archway, back door, baluster, balustrade,
   banister, barway, bulkhead, carriage entrance, cellar door,
   cellarway, door, doorjamb, doorpost, doorway, front door, gate,
   gateway, hatch, hatchway, hitching post, jamb, king post, lintel,
   milepost, mullion, porch, portal, porte cochere, post, postern,
   propylaeum, pylon, scuttle, side door, signpost, snubbing post,
   stanchion, standard, stile, storm door, threshold, tollgate, trap,
   trap door, turnpike, turnstile, upright