The Collaborative International Dictionary of English v.0.48:

Gab \Gab\, v. i. [OE. gabben to jest, lie, mock, deceive, fr.
   Icel. gabba to mock, or OF. gaber. See 2d Gab, and cf.
   Gabble.]
   1. To deceive; to lie. [Obs.] --Chaucer.
      [1913 Webster]

   2. To talk idly; to prate; to chatter. --Holinshed.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Gab \Gab\ (g[a^]b), n. [Cf. Gaff.] (Steam Engine)
   The hook on the end of an eccentric rod opposite the strap.
   See. Illust. of Eccentric.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Gab \Gab\, n. [OE. gabbe gabble, mocking, fr. Icel. gabb
   mocking, mockery, or OF. gab, gabe; perh. akin to E. gape, or
   gob. Cf. Gab, v. i., Gibber.]
   The mouth; hence, idle prate; chatter; unmeaning talk;
   loquaciousness. [Colloq.]
   [1913 Webster]

   Gift of gab, facility of expression. [Colloq.]
      [1913 Webster]

WordNet (r) 3.0 (2006):

gab
    n 1: light informal conversation for social occasions [syn:
         chitchat, chit-chat, chit chat, small talk, gab,
         gabfest, gossip, tittle-tattle, chin wag, chin-
         wag, chin wagging, chin-wagging, causerie]
    v 1: talk profusely; "she was yakking away about her
         grandchildren" [syn: yak, gab]

Moby Thesaurus II by Grady Ward, 1.0:

121 Moby Thesaurus words for "gab":
   accents, babble, babblement, balderdash, balls, bavardage, bazoo,
   bibble-babble, blab, blabber, blah-blah, blarney, blather, blether,
   blethers, bull, bullshit, bunk, bunkum, cackle, caquet, caqueterie,
   chaps, chat, chatter, chitter-chatter, chops, clack, clatter,
   comment, conversation, converse, crap, declaim, discourse, dither,
   drivel, elocution, embouchure, eyewash, flummery, gabble, garbage,
   gas, gibber, gibble-gabble, go on, gob, gobbledygook, gossip, guff,
   gush, haver, hogwash, hot air, idle talk, jabber, jaw, jaws, jowls,
   kisser, language, lips, mandibles, maw, maxilla, mere talk,
   moonshine, mouth, mug, mumbo-jumbo, mush, muzzle, natter, nonsense,
   nonsense talk, oral cavity, oral communication, palaver, parole,
   patter, piffle, poppycock, pour forth, prate, prating, prattle,
   premaxilla, prittle-prattle, ramble on, rapping, rattle, rattle on,
   reel off, rot, rubbish, run on, shit, speak, speaking, speech,
   spout, spout off, stuff and nonsense, talk, talk away,
   talk nonsense, talk on, talkee-talkee, talking, tittle-tattle,
   trap, twaddle, twattle, waffle, wag the tongue, words, yak,
   yakkety-yak, yakking, yap