The 2 gallon is included in the 16 gallon tank.
When the gauge level reach the red zone, the low fuel light comes up.
When the gauge level reach the bottom, you supposed have 2 gallon of "reserved" gas left, you actually burned 14 gallons of fuel from a full tank at this point.

But my experience is that the tank actually carries more than 16 gallons of gas. One time I drove beyond 16 gallons of gas according to my milage calculation, and I then was able to fill up more than 16 gallons (maybe 16.5 or almost 17 gallons if I remember correctly) of gas at the pump before the pump clicks. But I never ran out of gas, so I don't know the exact number.