Supply & Demand. So either we’d have to get a massive influx of gold into circulation (like we discovered a huge mountain made of pure gold, or we figure out how to mine it from asteroids cheaply), or we would need to have a massive shortage of wheat (probably more likely). If wheat was ever scarce enough that it was worth more than gold, I would imagine our planet would have to be in some major apocalyptic scenario, and I’m not particularly good at predicting those. Damn you Y2K….