The Collaborative International Dictionary of English v.0.48:

gibber \gib"ber\ (g[i^]b"b[~e]r), v. i. [imp. & p. p. gibbered
   (g[i^]b"b[~e]rd); p. pr. & vb. n. gibbering.] [Akin to
   jabber, and gabble.]
   To speak rapidly and inarticulately. --Shak.

   Syn: jabber.
        [1913 Webster]

Moby Thesaurus II by Grady Ward, 1.0:

24 Moby Thesaurus words for "gibbering":
   blabbering, blabbing, blathering, chattering, drone, droning,
   gabble, gabbling, gibber, jabber, jabbering, jibber, maundering,
   mouthing, mumble, mumbling, murmuring, mutter, muttering, prating,
   prattling, susurration, whisper, whispering