GFS (Game File Script) is a file type supported by Eternity, which can be loaded at the command line with the -gfs parameter. The file, written in human-readable text, contains a list of game (IWAD) files, patch (PWAD) files, BEX and other files which are to be loaded together. Like WADs, a GFS file can also be loaded by dragging them onto the Eternity executable or shortcut in Windows.
Its main purpose is to allow the user to group together a large list of files that need to be loaded together to play a mod, and is an alternative to using response files, shortcuts or batch files.
For more detailed reference information, see the Eternity documentation on GFS.