ET = expansion tank
WH = Water heater
You'd need to find somewhere else to attach a pressure gauge...the boiler SHOULD be at much less pressure than the domestic supply (typically 12-16#). A washing machine supply, or the drain on the WH, or an outside hose bib, or nearly anywhere with an adapter. Or, if you want it more permanent, you could get one with a NPT on it, and install a threaded T somewhere after the PRV and screw one in permanently. Or, if you're really interested, install two, one before and one after the PRV.