Anne Stevenson-Yang, a journalist and software executive who's worked and lived in China since 1985, and now a research director at an investment capital firm, has this to say about China's GDP: "actual Chinese GDP is probably a third lower than is officially reported"
"China is sinking into a deflationary recession that’s increasing in speed and may take some time to run its course. Investors have lost faith in the property market, which alone comprises about 20% of GDP, when taking into account the entire supply chain, from iron-ore production to construction to related financial services and appliance sales. Employment and wage compensation will suffer. Consumption will continue to suffer. There’s even an outside possibility that China’s economic miracle could end up in a fiery crash landing, if a surge in banking-system loan defaults outruns government regulators’ attempt to contain such a credit crisis and restore financial confidence."
http://www.barrons.com/articles/anne-stevenson-yang-why-xi-j...
"China is sinking into a deflationary recession that’s increasing in speed and may take some time to run its course. Investors have lost faith in the property market, which alone comprises about 20% of GDP, when taking into account the entire supply chain, from iron-ore production to construction to related financial services and appliance sales. Employment and wage compensation will suffer. Consumption will continue to suffer. There’s even an outside possibility that China’s economic miracle could end up in a fiery crash landing, if a surge in banking-system loan defaults outruns government regulators’ attempt to contain such a credit crisis and restore financial confidence."
She has a good video about China's economy here: https://www.youtube.com/watch?v=C2SStFt-k_A