Hay varias formas de lograrlo, intenta con este query:
SELECT ItmsGrpNam,Total,GBruta,Numdia,NumDiasMes,Pronostico FROM ( SELECT T3.[ItmsGrpNam], sum(T1.[TotalSumSy]) as Total, sum(T1.GrssProfSC) as 'GBruta', (sum(T1.GrssProfSC)/NULLIF(sum(T1.[TotalSumSy]),0))*100 as Margen, DAY(GETDATE()) as NumDia, DAY(EOMONTH(getdate())) as NumDiasMes, CAST(T5.U_VentasMTY as numeric) AS Pronostico FROM OITM T2 LEFT JOIN OITB T3 ON T2.ItmsGrpCod = T3.ItmsGrpCod LEFT JOIN [dbo].[@PRONOSTICOS] T5 ON T3.ItmsGrpNam = T5.Name LEFT JOIN INV1 T1 T1.ItemCode = T2.ItemCode LEFT JOIN OINV T0 ON T0.DocEntry = T1.DocEntry AND T0.[DocDate] >='20160501' AND T0.[DocDate] <='20160531' AND T1.TargetType <> '14' INNER JOIN OSLP T4 ON T0.[SlpCode] = T4.[SlpCode] AND T4.Memo='VentasMTY' GROUP BY T3.[ItmsGrpNam],T5.U_VentasMTY) x
Saludos y suerte,
JC: