SELECT t.*
     , SUM ( DECODE ( rn, 1, sum_ ) ) OVER ( ORDER BY gid )
  FROM (
          SELECT t.*
               , SUM ( amt ) OVER ( PARTITION BY gid ) sum_
               , ROW_NUMBER () OVER ( PARTITION BY gid ORDER BY num ) rn
            FROM t
) t

SELECT num, gid, amt
     , SUM(amt) OVER(ORDER BY gid) tot_amt
  FROM t
;

-- MSSQL : Self Join 이용
SELECT a.num, a.gid, a.amt
     , SUM(b.amt) tot_amt
  FROM t a
 INNER JOIN t b
    ON a.gid >= b.gid
 GROUP BY a.num, a.gid, a.amt
 ORDER BY gid, num
;