Welll, guys, all I know is what I have observed.
Gas engines have compression braking, even those old 7:1 compression ratio Model A 4 bangers. It is just the nature of the beast.
Diesel engines (at least the Cummins in my old Dodge 3500 and the little 4 banger in my older Datsun pickup) have negligible compression braking. The Dodge/Cummins however, had a Jacobs Rambrake installed on the engine, and when it was engaged there was a LOT of braking power! Since there was NO intake vacuum, it had a vacuum pump installed to supply vacuum to the cruise control, the Jake Brake, and the power brakes.
As for the throttle plate discussion, I have tried starting gas engines of various sizes by pushing or rolling down hills, and even with the throttle open there is a LOT of compression braking until the engine starts running (if it starts!).
My old Datsun diesel pickup had a throttle plate and venturi (the venturi created vacuum to control the injection pump via a diaphragm). With the throttle plate closed, it still had negligible compression braking.
I have no idea what the technical explanation of all this is, and don't really care. The observations are factual.
Oh, yeah, to answer the original quesation: No, there is no Jake brake or similar unit for gas engines. They don't need them. the inherent compression braking does the job just fine.
