metals

True - brass is unsuitable as de-zincification can occur, especially below the water line. Some societies won't certify boilers with brass fittings. As to identifying what it is, that's more difficult. Phosphor bronze should contain no zinc so if you lightly abrade the surface and apply a drop of dilute acid (e.g from battery) then if there's no copper colouration, it should be p-b. The composition of "gunmetal" is variable and might contain 2% zinc, but not always. Thus the acid test might work here also, Gun metal (like p-b) should tougher to scratch than brass. I'd suggest for any of the above tests you get a known sample of each and compare the results on your fittings with the test samples.