Gage pressure is defined as:

Gage pressure is defined as the pressure measured relative to the ambient atmospheric pressure. This means that when you measure gage pressure, you are determining how much pressure is in a system above the atmospheric pressure surrounding it. For example, if a tire gauge reads 30 psi, this indicates that the pressure inside the tire is 30 psi greater than the atmospheric pressure outside the tire.

This definition is critical in many engineering applications, where it is necessary to know the pressure differential that drives fluid flow or other processes rather than just the absolute pressure.

In contrast, total pressure refers to the absolute pressure including atmospheric pressure. Measuring pressure above absolute zero does not provide the context needed to define gage pressure. Lastly, while the pressure in an open container may be measured and could be gage pressure, that definition is too specific and does not encompass all scenarios of gage pressure measurement.