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

Group Code Recording
GCR

    (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.  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.  It was also supported on
   Amiga internal and external drives but only used for reading
   non-Amiga disks.

   Compare NRZI, PE.

   (2004-06-01)