I don't think the computer will cut the gasoline fully off. I think it will cut it down to about 10% ( compared to running fully on gasoline ) No real basis for the 10% , just a WAG from watching the vacuum gauges and feeling the power or the lack there of .
When these trucks are run hybrid with the minimum amount of gasoline it works best at a constant cruising speed and very slow movement of the throttle. Any quick action of the throttle , decelerating or accelerating while running on the minimum gasoline seems to throw the fuel/air ratio out of balance . .
When hybirding with a high % of gasoline quick movement of the throttle doesn't seem to matter . ( Pulling a hill with a loaded trailer or entering fast traffic with my work truck )
When I kill the fuel pump no petrol is pulled in the motor .
Edit . I found a video of my 8L truck on the big road hybird driving