So they have to buy $40 trillion worth of gold, and the number is increasing by several trillion a year. How much are they putting into it a year? If the number is not a lot over $2 trillion, they are getting further from the goal every year.
Let's say China commits to a 50% cross the board tax, on top of whatever taxes they are already paying. That would double the cost of all their exports to us, and would make them very uncompetitive, but this is just a hypothetical. After 5 years of absolute pain, they could in theory afford to do this.
And still we would be left with the question: why bother?