WordNet (r) 3.0 (2006):

gigabyte
    n 1: a unit of information equal to 1000 megabytes or 10^9
         (1,000,000,000) bytes [syn: gigabyte, G, GB]
    2: a unit of information equal to 1024 mebibytes or 2^30
       (1,073,741,824) bytes [syn: gigabyte, gibibyte, G,
       GB, GiB]

The Free On-line Dictionary of Computing (30 December 2018):

gigabyte
gig

    (GB or colloquially "gig") A unit of data equal to
   one billion bytes but see binary prefix for other definitions.
   A gigabyte is 1000^3 bytes or 1000 megabytes.

   A human gene sequence (including all the redundant codons)
   contains about 1.5 gigabytes of data.

   1000 gigabytes are one terabyte.

   See prefix.

   Human genome data content

(http://bitesizebio.com/articles/how-much-information-is-stored-in-the-human-genome/).

   (2013-11-03)