Source : Free On-Line Dictionary of Computing

Group Code Recording
     
         (GCR) A recording method used for 6250 BPI {magnetic
        tapes}.
     
        GCR typically uses a group of five bits of code to represent
        four bits of data, where the encoding ensures no more than two
        or three zeros occur in a row, and no more than eight or so
        ones occur in a row, where zeros represent an absense of
        magnetic change.
     
        GCR is also used on {Commodore Business Machines} {diskette}
        drives; the 4040, 8050, 154x, 157x and 158x series of 5.25"
        and 3.5" low and high density diskette drives used with 8-bit
        home computers circa 1977 to 1992.  Also used on {Amiga}
        internal and external drives.
     
        Compare {NRZI}, {PE}.
     
        (1997-08-29)