I have two thoughts on this one... or maybe one thought and one piece of advice. It's definitely a brass button with Gold gilt. Anything made of brass will start to show its original brass color if you scrub the heck out of it. And now the advice.... if you're finding buttons of this era on a site there's a chance you might turn up a button that has some value. The last thing you'd want to do with a valuable button would be to clean it to the point you cleaned that one. (until the brass shines through) If you were to do that to a button with any value you'd end up ruining it and also lowering the value substantially. Nice button find! Congrats!
Like most other responses...I too believe it's brass, I'd bet on it I'm that sure(and I don't gamble). Not knocking your find at all it's a cool find!! Doesn't make it any less of a find, just not solid gold.