Gunite is simply the result of a specific way of inline mixing concrete. It is a trade name for "dry gunned" concrete.

"Dry gun" means the cement and sand are injected into an air stream that is then conveyed to a nozzle. The nozzle operator then adds the water at the nozzle and has total control of the water-cement ratio. The delivery hose of the mix is generally quite light, as the hose is mostly filled with air containing the mix of cement and sand.

For a pool, you further plaster coat the "gunite". About 7 continuous days of brushing the plaster after the pool is filled with water will create a very smooth surface.

You would need to hold the "gunite" together with rebar to avoid cracks and leakage. A "gunite" lake will be very expensive to build... I'm not sure that it would even be possible to build a 20 acre concrete bottom lake that wouldn't eventually crack and hence leak...