Hi Terry,
it doesn't need to be one single product category, in case your Gross/Net classification correlates in a sufficiently natural* way with your existing Product Category classification, then assignment to the relevant Product Categories would offer a quite simple strategy to fulfull your requirement.
(*e.g. after a split of one existing category in "Parts - Gross" and "Parts - Net" or whatever would be feasible for you).
On the other hand, if the there is no reasonable correlation at all, then this approach might be too artificial, so the feasibility of this still quite simple approach depends on the actual data distribution that you have.
Best regards
Michael