A gauge is just a gauge, right? If so, can gauges that are marked for r12, r-22, r-502 accurately read r134a and r404 as well?

When it comes to pressure readings (located on the outer ring of the gauge), it doesn’t matter what type of gas is being measured. The difference is in the temperature scales (located on the inner rings of the gauge). These temperatures are based on the Pressure-Temperature data for the specific refrigerants. If you rely on your gauges for Pressure-Temperature data, it is important that you have a gauge that is marked for the refrigerant you are working with. If you don’t use the temperature rings, then any gauge will work for you.