The Yuan isn't convertible. It is kind of pointless to evaluate GDP in USD done by converting Yuan amounts to USD.I've noticed that for countries with similar nominal GDP per capita, the PPP to nominal ratio for China is among the lowest (if not the lowest).
The fact that China is experiencing deflation at roughly the same as everyone else is experiencing inflation is another sign that the Yuan is worth more than what it appears to be worth on the surface. Will probably only appreciate more as demand for Yuan continues to grow.