The Jargon File (version 4.4.7, 29 Dec 2003):

gillion
 /gil'y@n/, /jil?y@n/, n.

    [formed from giga- by analogy with mega/million and tera/trillion] 10^9.
    Same as an American billion or a British milliard. How one pronounces this
    depends on whether one speaks giga- with a hard or soft ?g?.


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

gillion

    /gil'y*n/ or /jil'y*n/ (From giga- by analogy with
   mega/million and tera/trillion) 10^9.

   Same as an American billion or a British "milliard".  How one
   pronounces this depends on whether one speaks giga- with a
   hard or soft "g".

   [Jargon File]

   (1995-03-17)