China GDP has been much underestimated for long time. Based on PPP, China GDP surpassed the States in 2014 (see IMF website), although China GDP is still smaller in terms of nominal rate. In other words, China currency has been underestimated.
I have a simple way to estimate the exchange rate between China Yuan and the US dollar on the basis of annual electricity production per capita (AEPPC). China AEPPC is about one third of the US counterpart, i.e. that one China Yuan should be also one third of the US dollar, given the physical law is right.
Therefore, China 2017 GDP was 84 trillion Yuan, equivalent to ~US $33 trillion ( = 86/2.6).