Those fittings that come with most mechanical oil pressure gauges are junk anyways. So it's probably not entirely your fault that it got all rounded off.
I don't know about that but a LOT of "hardware" nowadays, and brass fittings have all become suspect, is that the damn things are not US wrench sizes when they are supposed to be SAE fittings!!!!
When I "did" my brakes, I bought some fittings to plug the lines. I've got two "identical" 3/16 inverted flare unions, and neither one fits a U.S. wrench, unless you want to count a crescent.