If the Gateway 500S is compatible with the car, then both the 'single FOT' and 'dual FOT' version of it is compatible with your car.
The single FOT version Gateway has only one optical connector to install the device to the optical circle of the car. The dual FOT version has two optical connectors, one to connect the device to the optical circle of the car and a second one ("CDC side") to retain the factory CD changer.
So in case of the dual FOT you can keep the original 6CD changer of the car, by the single FOT you can't keep that. If you bought a dual FOT version although you don't have a CD changer in the car, that is not a problem, as you just won't use the second ("CDC side") optical connector of the Gateway then.