See if this works for you:
SELECT YourCol +1 FROM YourTable A
WHERE NOT EXISTS (SELECT 1 FROM YourTable B WHERE A.YourCol +1 = B.YourCol)
AND YourCol <> (SELECT MAX(YourCol) FROM YourTable)
NOTE: If you have a gap of 2 or more between numbers, you will have to run this query again, after inserting the initial set of missing numbers, and keep running the query until it returns no records.