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

global index

    (gid) The filename extension of a
   Windows 95 "global index" file.  .gid files are created by
   the help browser internal to Windows 95 (also available for
   other Windows versions) for WinHelp files (hlp), as well as
   for storing user preferences, such as window position.

   (1997-01-30)