I'm not sure if this will help you but it sounds as if you need a calendar table (I used a CTE to get the needed months). I hope this gets you closer to what you need.

WITH CTE AS(

SELECT TOP 12 DATEADD( mm, (ROW_NUMBER() OVER(ORDER BY(SELECT NULL)) -1) * -1, DATEADD( mm, DATEDIFF( mm, 0, GETDATE()), 0)) period

FROM sys.all_columns

)

SELECT *

FROM CTE

LEFT

JOIN GPByMonth Cur ON Cur.M = MONTH( period) AND Cur.Y = YEAR( Period)

LEFT

JOIN GPByMonth Prev ON Prev.M = MONTH( period)

AND Prev.Y = YEAR( Period) - 1

AND Cur.SPRSNSLN = Prev.SPRSNSLN

ORDER BY Cur.SPRSNSLN , period DESC