A pressure gauge is an instrument for measuring the pressure of gases and liquids. It usually consists of a closed coiled tube connected to the chamber or pipe in which pressure is to be sensed. As the pressure increases the tube will tend to uncoil. This motion is transferred through a link to a gear train connected to an indicating needle, which indicated the pressure on a circular scale at the front of the gauge. The input of a typical pressure gauge is either a hose barb or male pipe thread. In the case of male pipe threads, you will need to tap a hole.
Pressure gauges are made for different maximum pressures, though they all typically can measure from 0 psi and upwards. It's important to choose a gauge that can handle the desired pressure. A gauge made for much higher pressure than needed will work, but with lower resolution and accuracy.
Pressure gauges are mainly used to indicate chamber pressure on pneumatic and hybrid cannons, meter pressure on fuel meters, peak pressure in some hybrids (in conjunction with a check valve), and tank pressure on bulk tank regulators and compressors. Some pressure regulators have built-in gauges for output pressure, negating the need for a separate gauge.
The pressure indicated by the gauge is measured in comparison to atmospheric pressure, if this needs to be indicated, "gauge" or "g" is added to the unit of measure, for example "100 psi gauge" or "100 psig".