That's not evidence of a decline, see http://www.wolframalpha.com/input/?i=US+real+GDP+over+time. That would seem to indicate that other countries are catching up, not that the US is declining. Also, in real terms, it doesn't look like the narrowing of the gap is accelerating (http://www.wolframalpha.com/input/?i=US+minus+China%2C+US+mi...).