The 3/8" size refers to the "standardized" measuring system employed by European espresso machine manufactures that started producing these machines using the British Standard Pipe size/style fittings from their inception. To make it easier to determine/understand, measure the outside diameter of your portafilter spout fitting. A 3/8" fitting will measure 5/8" on the outside (the male fittings inside diameter is the true source of the specification, but not in the case of the portafilter bodies, as the opening needs to be larger for the coffee to go through, just in the case of the other male fittings on the machines).

The gauge reads in one bar increments from 0 to 25 bar.