Greenplum database at it's core uses much of the same code as a Postgres database. Therefore, the command you want is the VACUUM command. From the docs at http://gpdb.docs.gopivotal.com/4300/pdf/GPDB43RefGuide.pdf:
VACUUM reclaims storage occupied by deleted tuples. In normal Greenplum Database operation, tuples that are deleted or obsoleted by an update are not physically removed from their table; they remain present on disk until a VACUUM is done. Therefore it is necessary to do VACUUM periodically, especially on frequently-updated tables.
Also, if you are altering a significant number of rows, then you may want to use VACUUM ANALYZE to allow the statistics for the table to be updated for better query planning.