hard to say as I do not know what your @activity formula is doing but in general for this sort of thing you can use variable formulas or Running Totals with an evaluation formula.
{VW_CUST_ORDER.DESCRIPTION} = "GA67 GALLIUM CITRATE"
Sometimes you can also use another formula to zero out the rows you do not want to include and then sum that formula
if {VW_CUST_ORDER.DESCRIPTION} = "GA67 GALLIUM CITRATE" then @activity