"China’s sinking stock market reached an unwelcome milestone, with the
Shanghai Composite Index closing at the lowest level since 2014,
erasing the last traces of its recovery from a boom that turned into a
$5 trillion bust.
The Shanghai gauge dropped 1.1 percent to 2,651.79, below its January
2016 bottom. Back then, officials had just introduced and then hastily
scrapped a disastrous circuit-breaker program as they grappled with
one of the market’s worst-ever routs."
They use gauge to avoid repeating the term index or. Acceptable terminology is: Shanghai Composite Index, the Shanghai index or the Shanghai gauge. A gauge means something that measures something. But you would not append gauge to: the Shanghai Composite Index.
Here is another example used for the New York Stock Exchange (another gauge):
TITLE in USA Today: Dow's biggest 2-day drop since 2016 puts
investors on edge as stock gauge briefly falls 400 points
Any measure of marketplace activity where shares or other financial instruments are traded on an exchange, including commodities, can be termed a gauge, meaning measure of the average daily volume of trading (purchase and sale of financial instruments).