Problem with this idea is,
 - It will open up a huge black market of gold dealers who'd just avoid the blokchain way.Such markets will buy/sell gold illegally and for a much higher price.
 - This may also impact the prices of gold as an asset.Imagine what if the huge chunk of gold is dealt with illegally.

Moreover certificates on the Gold are country wise unique aren't they ? On the blockchain,they all will follow the princile of distributed ledger ?