The external engine braking mechanisms used on diesel engines are there to compensate for the lack of compression braking that gas engines inherently have by design. The basic difference is that gas engines restrict the inlet air flow when you lift the throttle, while diesel engines restrict the fuel flow. That air flow restriction and resulting "compression braking" is what the external "Jake" type braking systems on diesels duplicate. To get the full compression braking effectiveness on a gasser, just drop it out of overdrive and downshift as needed for the conditions.
