What brand gauge is it? Every Autometer I've ever gotten has the -4 AN adapter included with the gauge. It goes on the pipe-thread sticking out of the back of the gauge. The fitting should be easy to find at a speed shop, or you can look it up on Autometer's site and plug the p/n into Summit's site. You'll need a length of -4 AN braided steel hose with.
You are mounting that outside the car, right? If not you'll need an isolator, too.
Jim