G-Max has higher water in the tank, so it uses the red flapper that drops quicker.
Power Gravity has the lower level in the tank, uses the blue flapper that stays up longer.
The water measured, is the total of the water that leaves the toilet during a flush. Even if you see the tank drop, you still can't count fast enough, to account for the immediate filling of the tank and bowl by the fill valve. It fills near the bottom of the tank, and from the black tube leading into the overflow.
And that's the way "all" 1.6 gallon toilets work.
Even the old 3.5's don't use much water for the flush itself, that's why some plug easier than the new ones. The old toilets tend to overfill the bowl, long after the flush has happened.