Is 1.28gpf Enough Water?

I have installed a 1.28 Toto Eco Drake, I have seen it flush and I don't have doubts about its flushing ability despite the minimal water usage. I'm neither a plumber nor an engineer, so I always have thought, is 1.28 gallons of water enough to push "it" out to the main sewer or is it just sitting in the pipe under my house waiting in queue for the succeeding flush?

Your right it may not be enough.

There are a lot of factors that determine flush.
The design of the Drake is that it flushes like a waterfall, a large volume of water at one time not like the old toilet which slowly swirl around the bowl and out.
If you are installing the toilet in the basement you may not have enough of a fall (gravity) to get the waste flowing fast enough and it may stop part way through the pipe.
If you have a long run of pipe or if the pipe is not sloped enough or if the diameter of the pipe is small...

Not getting to the end is fairly common...it would only be a problem if there was no flow from anything else for a long time or you had improperly installed (bad pitch) pipes and it dried into a lump...but, it's very humid down there. That would take a long time.

In order for a flush of the toilet to move the solid waste all the way to the city line in one flush it would probably have to be a very short run.

Solid waste is often moved in steps to the street and relies heavily on other flows to wash the line clear such as baths, showers, dish water, waching machine water etc.

In a line that is badly worn and leaking into the soil if there is not a lot of other flows a dry pack clog may result...