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

Group-Sweeping Scheduling

    (GSS) A disk scheduling strategy in which
   requests are served in cycles, in a round-robin manner.  To
   reduce disk arm movements ("seeking"), the set of streams is
   divided into groups that are served in fixed order.  Streams
   within a group are served according to "SCAN".

   If all clients are assigned to one group, GSS reduces to SCAN,
   and if all clients are assigned to separate groups, GSS
   effectively becomes round-robin scheduling.  The service order
   within one group is not fixed, and a stream may in fact be
   first in one cycle while last in the next.  This variation has
   to be masked by extra buffering but whereas SCAN requires
   buffer space for all streams, GSS can reuse the buffer for
   each group and effect a trade-off between seek optimisation
   and buffer requirements.

   (1995-11-12)