GMs new 6.2 di seems to get great mpg

I wouldn't think Direct Injection would affect engine braking at all (gasoline application). In fact, most manufacturers increase compression ratio when they do move on to Direct engine. If anything, it should have a higher engine-braking effect. Gasoline DI engines still have the throttle plate and all the other bits associated with gas motors, they still have the pumping losses that make them less efficient than diesels.