I suggest taking it into the boutique yourself and having them authenticate it. Or maybe you can have it authenticated on the Chanel forum.. anyway, that's awful, hope someone can help you figure that out
My mother has a GST that she carries EVERY DAY for a very long time and the gold hw is turning silver... But do visit the authenticication thread for more info...
My reissue with gold hw is currently in Paris having the chain and front lock replaced as some parts were turning silver! But like others have said I would definitely recommend posting it in the authenticate thread.