Set up the algebraic formula of PG = PS - PA. The variable PG stands for the gauge pressure you want to calculate. PS represents system pressure, or the natural force a particular system, such as a tire or veins, exerts. PA represents atmospheric pressure, or the pressure exerted by the surrounding air.
Measure the system pressure of a particular system using a system pressure gauge, and insert that value into your algebraic formula. Then, measure the atmospheric pressure of that same system with an atmospheric pressure gauge, and insert that value into the formula as well. There are a variety of different types of gauges to measure these values, so follow the instructions for your particular gauge.
Make sure that the values of both the system pressure and atmospheric pressure are in the same unit. Pressure is usually measured in pascals, but large units are sometimes converted into units such as kilopascals. If the values are different, use a conversion chart to convert one unit value into the same as the other. Then, subtract the atmospheric pressure from the system pressure to find the gauge pressure.