Hi Aaron,
Please try below.
SELECT T2.[WhsName], T0.[DocTotal], T0.[GrosProfit], (T0.[GrosProfit] / T0.[DocTotal] ) * 100 AS GP_Percent FROM OINV T0 INNER JOIN INV1 T1 ON T0.[DocEntry] = T1.[DocEntry] INNER JOIN OWHS T2 ON T1.[WhsCode] = T2.[WhsCode] WHERE T2.[WhsCode] =[%0]
Hope this help 
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Regards::::
Atul Chakraborty