What should be clarified about VGPMX, as well as what B. Bernstein actually advocates, is that precious metal equities shouldn't be confused with owning physical metals, or other commodities. Rephrased, VGPMX actually owns companies/is 100% stocks (not counting the few percent of cash it keeps on hand). It also owns some other non-metals related companies (like fertilizer companies) but still, actual companies, not raw commodities.
And as is well known by even novice investors, stocks have a considerably stronger long-term performance/track record than just precious metals commodities (bullion), or commodities in general. I suspect this has something to do with the fact that (successful) companies tend to have something called earnings and profit, and tend to actually make/supply things for consumers. In any event, stocks are a common core component of a portfolio. Commodities, if even owned at all, are properly referred to as alternative investments.
Precious metal equities were recently brutalized in the market, far worse even than the metals themselves. So if you like buying low, now may be your chance to overweight this sector of the Stock market. If you prefer Morningstar's groupings, its a subset of the basic materials Stock sector.