If you have a simple gravity system, how old is it and how often has it been pumped? 

But rather, since water seeks the path of least resistance and is subject to the laws of gravity, it will naturally collect in low spots.I was just on a project this year in which, despite having a floor drain, the concrete was not sloped properly to it and so water puddled elsewhere in the low spots of the basement.

I ran it for several years prior to that with window units & old gravity Oil Furnaces (what a PITA they were).

Why would you put a back flow preventer on a gravity flow system?

Hey small park like that is probably a standard gravity fed system. 

Most did not understand the full gravity of the home loan and buying process and due to that I had to redo a lot of the work myself since the departments in NACA do not have efficient systems of communication.

A quick search shows both pressure, and simple gravity which yours was.

The next thing I've got to ask if it's a monoflow or older, modified gravity-fed system.

Looks like the shed was secured in place by gravity.

Some systems work mainly on gravity, others have dual tanks where one has a pump that can need replacing every 5 years.