If this were silver that was copper plated, I would tell you to add
some hydrogen peroxide to some pickle and repickle the item.
However, you are using a bi metal and the gold may have alloyed into
the base metal. If you try the hydrogen peroxide and pickle mixture,
watch it closely. It's a very strong pickle and will etch solder if
the item stays in it to long. I don't measure amounts and find the
it works in five minutes or so.