Equity in a gold mining company is a leveraged play on gold: since the company has debt, the equity will react more than the price of gold (on both the upside and the downside) with a floor at 0. So he's purchased a call option on gold.
The gold mining company can generate future incoming cashflows, gold doesn't.
The equity in the gold company is a call on the gold in the ground: the cash flows of the company come from extracting this gold. Getting a bit more detailed, the leverage comes from 2 elements: (a) operating leverage i.e. company generates more revenue with same fixed operating costs and (b) financial leverage i.e. company generates more net operating income with same fixed interest costs. The basic point is that the cash flows all come from extracting gold, so the equity is a derivative on this underlying.