Vb or the net internal volume of the box or chamber. This is the volume that remains after subtracting the volume inside the box that is displaced by internal objects like the driver, vent and bracing. Gross would be for volume prior to the driver displacement, port displacement. Think of it in terms of your paycheck. Gross is prior to taxes, Insurance, 401k, etc. Net is after they take stuff out of the equation (paycheck) that you are left with.
Just call and ask what the Box VB (or net volume) is, if it is 2 cubes per driver, then that 4.5 cu ft box will be your ticket. Might as well ask them what the driver displacement is too. I'm guessing between .2 and .25 cubic ft. Once you get that info, we can look at the correct box configuration that GP offers to suit your tuning needs..