GIMP uses plugins for reading and writing all
      file formats except XCF. These plugins may use external libraries or
      programs. For example, GIMP does not directly support
      PDF and
      PostScript. Instead,
      for reading (or writing) PDF (file extension
      .pdf) and PostScript files
      (file extension .ps or
      .eps), GIMP
      requires a powerful free software program called
      Ghostscript.
    
Linux distributions almost always come with Ghostscript already installed (not necessarily the most recent version). On Windows Ghostscript[GHOSTSCRIPT] is included in the installer so you don't need to install it separately.