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.
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   2. To talk idly; to prate; to chatter. --Holinshed.
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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.
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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.]
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   Gift of gab, facility of expression. [Colloq.]
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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]