It depends on why you are checking gastric residual. A pt with that much residual obviously isn't moving gastric content through his GI tract. The NG should probably be connected to low wall suction so that the residual is removed over hours, instead of pulling 1200cc out all at once. I would expect the pt to have peripheral IV fluids infusing for hydration and to replace volume loss and maintain their blood pressure.
I would guaiac the gastric residual but I can't think of any reason to keep it, especially if the doctor is informed and says to discard it. You certainly would not put the withdrawn gastric content back into the patient via the NG.