Mr. Wizard, you got things a little backwards....
Gas engine DO have natural engine braking, closing the throttle restricts the airflow through the engine, causing it to resist turning (engine braking)
Diesels have no natural engine braking at all (because there is no throttle plate to restrict airflow)take your foot off the gas (cuts fuel only) and it just freewheels.
Diesels get an addon engine brake (usually a pacbrake, which is a valve that closes off the exhaust, or these days with variable vane turbos, the brake is built into the turbo) to provide the engine braking that gassers get naturally (and a lot more of it usually)
In all cases, managing the transmission (manually is fine, or automatically with a tow/haul. Diesels have transmission management built into the pacbrake system)helps keep the speeds down.
