1) In a Unix system, a GID (group ID) is a name that associates a system user with other users sharing something in common (perhaps a work project or a department name). It's often used for accounting purposes. A user can be a member of more than one group and thus have more than one GID. Any user using a UNIX system at a given time has both a user ID (UID) and a group ID (GID).
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2) In Windows 95, a file with a ".gid" suffix is a global index file. Windows 95 sometimes creates these files to hold information about a help file (the global index file has the same name as the help file but with the .gid suffix) and stores them as hidden files in the same directory as the help files.