To backup a partitioned table, it is suggested that you use gpcrondump instead of gp_dump. When using gp_dump -t xxx.tablename (parent table name), the output file will include no data. To avoid this, gp_dump -t requires that you include all the partitionsname. gpcrondump works well only if the parent table name is provided in the command.
gp_dump is no longer supported since Greenplum v4391. Using gpcrondump for any backup operation is thus better.
Please refer to this link for more information regarding gp_dump.
For more information about gpcrondump, refer to this link.