Also don't use the term "gas" this is an international forum and a substantial proportion of readers think "gas" refers to a gas.
(Also the use of US customary units is deprecated, but you can't do anything about that, its your teacher/educational system that is at fault on that (and NASA apparently)).
The force on a horizontal area element is the weight of the fluid above it ( where is the height of fluid above the area element), and as pressure is isotropic this is also the force on a vertical area element . So to get the pressure on an end face you integrate this over the face and divide by the area of the face.
Note: the formula for the weight I gave is for a consistent set of units and is the acceleration due to gravity, you will need to sort out the units of force/mass for US customary units for yourself. In SI units this weight will be in Newtons