From a DAU perspective, the approach you suggest, "... purchas[ing] each country's requirement separately using the same GPC and identifying the country case code and line in each order" would appear to provide sufficient documentation to 'split the requirements' between the set of FMS customer nations that are separately acquiring the same piece of equipment.  The additional effort that would be required to establish a separate GPC for each customer country does not appear to be needed or justified since you would establish/document 'traceability' for each separate GPC purchase order you place on behalf of each FMS customer country to meet that country's FMS case requirements using the procedures described in the first sentence.