Worst case scenario is something like, every dollar spent mining gold goes into diesel for machines moving dirt to find the gold, which amounts to something like 27 billion gallons of diesel per year, which amounts to something like 275 million metric tons of CO2 per year. Which would, in fact, be something like 7.5x more CO2 per year than BTC.

I'd put my estimate at something like 30-150 megatons of CO2/year for gold, just because I'd be surprised if they spent less than 10% of the cost of mining gold on fuel, or more than 50%. So I'd be neither surprised if gold generated less CO2 / year than Bitcoin, or if it generated like 4x more.