The Collaborative International Dictionary of English v.0.48:

Gauche \Gauche\ (g[=o]sh), n. [F.]
   1. Left handed; hence, awkward; clumsy.
      [1913 Webster]

   2. (Geom.) Winding; twisted; warped; -- applied to curves and
      surfaces.
      [1913 Webster]

   3. Lacking grace and perceptivity in social situations;
      crude; tactless; socially inept.

   Syn: graceless; unsophisticated.
        [PJC]

   4. (Chem.) Not planar; -- of molecules or molecular
      conformations.
      [PJC]

WordNet (r) 3.0 (2006):

gauche
    adj 1: lacking social polish; "too gauche to leave the room when
           the conversation became intimate"; "their excellent
           manners always made me feel gauche" [syn: gauche,
           graceless, unpolished]

Moby Thesaurus II by Grady Ward, 1.0:

80 Moby Thesaurus words for "gauche":
   all thumbs, awkward, blankminded, blunderheaded, blundering,
   boorish, bumbling, bungling, butterfingered, callow, careless,
   clownish, clumsy, clumsy-fisted, crude, cumbersome, dumb, empty,
   empty-headed, fingers all thumbs, fumbling, gawkish, gawky,
   graceless, green, groping, halting, ham-fisted, ham-handed,
   heavy-handed, hulking, hulky, ignorant, inane, inelegant, inept,
   inexperienced, innocent, know-nothing, left-hand, left-handed,
   loutish, lubberly, lumbering, lumpish, maladroit, naive, nescient,
   oafish, ponderous, raw, simple, sloppy, stiff, strange to,
   tentative, unacquainted, unapprized, uncomprehending, unconversant,
   uncouth, unenlightened, unfamiliar, ungainly, ungraceful, unhandy,
   unhappy, unilluminated, uninformed, uninitiated, unintelligent,
   unknowing, unpolished, unposted, unripe, unsure, unversed,
   unwieldy, vacuous, wooden