Summing over a calculated field of concatenated strings (split acct #s)

Question 1

Am assuming the error is SQL0206 - Column or global variable AJAccount not found.

If true, the problem is that the GROUP BY clause is looking for a column in the database table, not a derived column in the SELECT clause. Try putting the entire derived column in your GROUP BY:

select...
where...
group by right(trim (trailing from (concat('00',cast(T1."GMFUND" as Char(8))))),3) || '-' || right(trim (trailing from (concat('00',cast(t1."GMDPT" as Char(8))))),2) || right(trim (trailing from (concat('00',cast(T1."GMDIV" as Char(8))))),2) || '-' ||  right(trim (trailing from (concat('00',cast(T1."GMSTAB" as Char(8))))),2) || right(trim (trailing from (concat('00',cast(T1."GMSTAS" as Char(8))))),1) || '.' || right(trim (trailing from (concat('00',cast(T1."GMELM1" as Char(8))))),1) || 
right(trim (trailing from (concat('00',cast(T1."GMELM2" as Char(8))))),1) || '-' || right(trim (trailing from (concat('00',cast(T1."GMOBJ" as Char(8))))),2) 
order by 1

Question 2

A simplified example using a common-table-expression:

WITH gm310ap AS (SELECT 
    RIGHT('00'||DIGITS(gmfund),3) || '-' ||
    RIGHT('0'||DIGITS(gmdpt),2) || 
    RIGHT('0'||DIGITS(gmdiv),2) || '-' || 
    RIGHT('0'||DIGITS(gmstab),2) ||
    RIGHT(DIGITS(gmstas),1) || '.' ||
    RIGHT(DIGITS(gmelm1),1) ||
    RIGHT(DIGITS(gmelm2),1) || '-' ||
    RIGHT('0'||DIGITS(gmobj),2) AS AJAccount,
    COALESCE(gmdamt,0) - COALESCE(gmcamt,0) AS Trans_Total
FROM "HTEDTA"."GM310AP"
WHERE gmapyr = 2014)
SELECT AJAccount, SUM(Trans_Total)
FROM gm310ap
GROUP BY AJAccount

Documentation for the scalar functions:

Question 3

If the code you have shown is exactly what you ran, then your query has a simple syntax issue. You said GROUP BY 'AJAccount' which is a string literal. Double quotes ("AJAccount") would make it a case sensitive column reference. However, when you said as AJAaccount you did not use double quotes, so the name is not case sensitive and so you do not need the quotes in the GROUP BY.

Assuming your columns are at least as long as the portions you are trying to extract, I would try the following query:

SELECT (  RIGHT(DIGITS(gmfund),3) ||'-' 
       || RIGHT(DIGITS(gmdpt),2)
       || RIGHT(DIGITS(gmdiv),2) || '-'
       || RIGHT(DIGITS(gmstab),2)
       || RIGHT(DIGITS(gmstas),1) || '.'
       || RIGHT(DIGITS(gmelm1),1)
       || RIGHT(DIGITS(gmelm2),1) || '-'
       || RIGHT('0'||DIGITS(gmobj),2)
       ) as AJAccount,
       sum(gmdamt) - sum(gmcamt) AS Trans_Total
    FROM "HTEDTA"."GM310AP"
    WHERE gmapyr = 2014
    GROUP BY AJAccount

My guess is that there may be plenty of other queries that would use this concatenated account number. If this is the case, you might consider creating a view to store this logic for use elsewhere, perhaps like this:

CREATE OR REPLACE VIEW HTEDTA.GM310AP_AC
  SELECT (  RIGHT(DIGITS(gmfund),3) ||'-' 
         || RIGHT(DIGITS(gmdpt),2)
         || RIGHT(DIGITS(gmdiv),2) || '-'
         || RIGHT(DIGITS(gmstab),2)
         || RIGHT(DIGITS(gmstas),1) || '.'
         || RIGHT(DIGITS(gmelm1),1)
         || RIGHT(DIGITS(gmelm2),1) || '-'
         || RIGHT('0'||DIGITS(gmobj),2)
         ) as AJAccount,
         gmdamt,
         gmcamt      -- include any other useful columns
    FROM HTEDTA.GM310AP

Once that has been created, you can write queries such as

SELECT AJAccount,
       SUM( IFNULL(gmdamt,0) - IFNULL(gmcamt) ) as Trans_Total
  FROM HTEDTA.GM310AP_AC
  GROUP BY AJAccount
  ORDER BY AJAccount

Or

WITH Q AS
( SELECT AJAccount,
         SUM(gmdamt) as TotD,
         SUM(gmcamt) as TotC
  FROM HTEDTA.GM310AP_AC
  GROUP BY AJAccount
)
SELECT AJAccount, 
       TotD - TotC   as Trans_Total
  FROM Q
  ORDER BY AJAccount

P.S. You only need the IFNULL if those columns are nullable. This is traditionally rather rare on DB2 for i, so if you check this, you might simplify things a bit more for yourself.