If you want to exclude 0 values also, then you have to provide this constraint in your query:
SELECT DISTINCT
ProjectNr,
SUM(GWVS) - SUM(GWVS2) AS GWValue1,
SUM(ABVS) - SUM(GWVS2) AS GWValue2,
SUM(TSVS) - SUM(GWVS2) AS GWValue2
FROM TBL1
WHERE RB IN (North)
GROUP BY ProjectNr
HAVING
(SUM(GWVS) - SUM(GWVS2)) <> 10
AND (SUM(GWVS) - SUM(GWVS2)) <> 0