WITH t (id, g_seq, l_seq, g_nm, g_order_seq, item_order_seq) AS
(
          SELECT '00000002',   16, NULL, 'test1',    1, NULL FROM dual
UNION ALL SELECT '00000002',   15, NULL, 'test2',    2, NULL FROM dual
UNION ALL SELECT '00000002',   17, NULL, 'test3',    3, NULL FROM dual
UNION ALL SELECT '00000002', NULL,   18,    NULL, NULL,    3 FROM dual
UNION ALL SELECT '00000002',   15,   17,    NULL, NULL,    2 FROM dual
UNION ALL SELECT '00000002',   15,   16,    NULL, NULL,    1 FROM dual
)
SELECT *
  FROM t
 ORDER BY id
     , MIN(g_order_seq) OVER(PARTITION BY id, g_seq) NULLS LAST
     , item_order_seq NULLS FIRST
;