What is the difference btw gauge and atmosperic pressure?

Gauge pressure measures pressure relative to the ambient atmospheric pressure, meaning it can be positive or negative depending on whether the measured pressure is above or below atmospheric pressure. In contrast, atmospheric pressure is the pressure exerted by the weight of the air in the Earth's atmosphere at a given location and can be considered a baseline reference, typically around 101.3 kPa at sea level. Thus, gauge pressure excludes atmospheric pressure, while absolute pressure includes it.