the serial cable that comes with the unit is more for programing/virtual control etc.
It *can sometimes* be used to hook up a serial GPS, however *most* GPS units will need another cable (a null modem adapter / gender changer) in order to hook the cables up together.
It will all depend on what cable is coming out of your GPS / what hookup your GPS currently has.
But most people do have to buy another GPS cable, or at least a null modem adapter/gender changer.