WITH detail_table AS
(
SELECT 1 detail_seq, 1 grp_seq, 'AG0000001' grp_id, '카테고리 > 전자기기' grp_path, 'I' grp_gubun FROM dual
UNION ALL SELECT 2, 1, 'AG0000001', '카테고리 > 전자기기 > 컴퓨터'  , 'E' FROM dual
UNION ALL SELECT 3, 1, 'AG0000001', '카테고리 > 전자기기 > 노트북'  , 'I' FROM dual
UNION ALL SELECT 4, 1, 'AG0000001', '카테고리 > 전자기기 > 외장하드', 'I' FROM dual
)
-- 1. Group By, Min(Decode)
SELECT grp_seq
     , grp_id
     , rn
     , MIN(DECODE(grp_gubun, 'I', grp_path)) grp_path_I
     , MIN(DECODE(grp_gubun, 'E', grp_path)) grp_path_E
  FROM (SELECT grp_seq, grp_id, grp_gubun, grp_path
             , ROW_NUMBER() OVER(PARTITION BY grp_seq, grp_id, grp_gubun ORDER BY detail_seq) rn
          FROM detail_table
        )
 GROUP BY grp_seq, grp_id, rn
 ORDER BY grp_seq, grp_id, rn
;
-- 2. PIVOT
SELECT *
  FROM (SELECT grp_seq, grp_id, grp_gubun, grp_path
             , ROW_NUMBER() OVER(PARTITION BY grp_seq, grp_id, grp_gubun ORDER BY detail_seq) rn
          FROM detail_table
        )
 PIVOT (MIN(grp_path) grp_path FOR grp_gubun IN ('I' i, 'E' e))
 ORDER BY grp_seq, grp_id, rn
;

WITH MASTER_TABLE AS (
    SELECT 1 AS GRP_SEQ, 'AG0000001' AS GRP_ID FROM DUAL
), DETAIL_TABLE AS (
    SELECT 1 AS DETAIL_SEQ, 1 AS GRP_SEQ, '카테고리 > 전자기기' AS GRP_PATH, 'I' AS GRP_GUBUN FROM DUAL
    UNION ALL
    SELECT 2 AS DETAIL_SEQ, 1 AS GRP_SEQ, '카테고리 > 전자기기 > 컴퓨터' AS GRP_PATH, 'E' AS GRP_GUBUN FROM DUAL
    UNION ALL
    SELECT 3 AS DETAIL_SEQ, 1 AS GRP_SEQ, '카테고리 > 전자기기 > 노트북' AS GRP_PATH, 'I' AS GRP_GUBUN FROM DUAL
    UNION ALL
    SELECT 4 AS DETAIL_SEQ, 1 AS GRP_SEQ, '카테고리 > 전자기기 > 외장하드' AS GRP_PATH, 'I' AS GRP_GUBUN FROM DUAL
)
 SELECT
     CASE WHEN A.GRP_SEQ IS NULL THEN B.GRP_SEQ ELSE A.GRP_SEQ END AS GRP_SEQ,
     CASE WHEN A.GRP_ID IS NULL THEN B.GRP_ID ELSE A.GRP_ID END AS GRP_ID,
     A.INCLUDE_NM,
     B.EXCLUDE_NM
 FROM (
         SELECT
             MT.GRP_SEQ, MT.GRP_ID,
             DT.GRP_GUBUN,
             DECODE(DT.GRP_GUBUN, 'I', DT.GRP_PATH) AS INCLUDE_NM,
             DECODE(DT.GRP_GUBUN, 'E', DT.GRP_PATH) AS EXCLUDE_NM,
             ROW_NUMBER() OVER (ORDER BY DT.DETAIL_SEQ DESC) AS ROW_NUM
         FROM DETAIL_TABLE DT
         INNER JOIN MASTER_TABLE MT ON DT.GRP_SEQ = MT.GRP_SEQ
         WHERE DT.GRP_GUBUN = 'I'
        ) A
 FULL OUTER JOIN (
     SELECT
         MT.GRP_SEQ, MT.GRP_ID,
         DT.GRP_GUBUN,
         DECODE(DT.GRP_GUBUN, 'I', DT.GRP_PATH) AS INCLUDE_NM,
         DECODE(DT.GRP_GUBUN, 'E', DT.GRP_PATH) AS EXCLUDE_NM,
         ROW_NUMBER() OVER (ORDER BY DT.DETAIL_SEQ DESC) AS ROW_NUM
     FROM DETAIL_TABLE DT
              INNER JOIN MASTER_TABLE MT ON DT.GRP_SEQ = MT.GRP_SEQ
     WHERE DT.GRP_GUBUN = 'E'
     ) B ON A.GRP_SEQ = B.GRP_SEQ AND A.ROW_NUM = B.ROW_NUM
;

자문 자답이지만 하다보니 이렇게 해도 답이 나오네요!

테이블 구조가 조금 이상하네요. 정규화 되어 있지 않은 느낌입니다.
grp_seq 가 키라면? grp_id 는 상세테이블에 있으면 안되는 거죠.
grp_id 가 상세테이블에 있다면, 마스터 테이블과의 조인이 불필요하구요.
작성하신 쿼리의 순번 부여하는 부분은 Partition by 구문이 필요해 보입니다.