You can probably step the gauge out a little bit on a longer nipple and use a more available one. You might need to step the threads up or down. For that matter you can get the whole regulator and water trap for abouot $12 from Amazon. I bought a Point Zero to mount to the side of my paint booth, leaving the compressor regulator cranked up, fine tune at the booth. Water separates better the further from the compressor you get. In 1/1 our separators were 35 ft from the compressor room lol.
Well till you get a gauge just shoot by flow is all, set your regulator for good paint flow, you don't really need to know the pressure, it's either flowing right or it isn't.