They wont be mining the entire asteroid at once and releasing all of the gold into the market at once. It would take decades to get even a fraction of it mined. Then the is the trillions it will cost in the long term to mine for the gold. It could bring down the cost of gold, but it wont make it drop like a rock. Collectors would also go after Earth gold, so there are possible new markets to add as well.

You could always sell your bullion before they start if you're concerned. It's going to happen. Gold and many other rare (on Earth) metals will be mined in space. The metal markets will change and adapt. Investors should be aware if they plan on living that long