[overloaded]

podrias hacerlo así:

Código SQL [-]
SELECT 
(
SELECT 
SUM(GL20000.DEBITAMT) - SUM(GL20000.CRDTAMNT)
FROM
  GL20000
  INNER JOIN dbo.GL00100 ON (GL20000.ACTINDX = dbo.GL00100.ACTINDX)
  WHERE
  GL00100.MNACSGMT = '51504010' and
  GL20000.TRXDATE BETWEEN '20100101' and '20100131'
) AS ENERO,
(
SELECT 
SUM(GL20000.DEBITAMT) - SUM(GL20000.CRDTAMNT) 
FROM
  GL20000
  INNER JOIN dbo.GL00100 ON (GL20000.ACTINDX = dbo.GL00100.ACTINDX)
  WHERE
  GL00100.MNACSGMT = '51504010' and
  GL20000.TRXDATE BETWEEN '20100201' and '20100228'
) AS FEBRERO,
(
SELECT 
SUM(GL20000.DEBITAMT) - SUM(GL20000.CRDTAMNT)
FROM
  GL20000
  INNER JOIN dbo.GL00100 ON (GL20000.ACTINDX = dbo.GL00100.ACTINDX)
  WHERE
  GL00100.MNACSGMT = '51504010' and
  GL20000.TRXDATE BETWEEN '20100301' and '20100331'
) AS MARZO,
.....
FROM DUAL