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Sorry.. It's not GDAX (that's an exchange). I looked it up and there are several but it seems more like an ETF but MNTP looks promising:
Screen Shot 2018-02-23 at 9.37.20 AM.png

I guess that 1 coin is worth 0.00226 oz of gold. This would mean they physically have to hold almost 4000 oz of gold to cover the current circulating supply of coins and 22,000 oz of gold to cover the total supply so mostly ETFs.