You have to make the coin very flat and laser cut the blockchain id into it do it and seal it somehow. Then sell the physical gold as a token and those tokens can have physical and virtual ownership on the blockchain. Until you register all the gold on the market on the blockchain. The physical owner can sell the virtual token to a buyer and to receive payment a smart contract will be created and the token will be in escrow till the physical token is received by the buyer or centralized escrow facility who will verify receipt to blockchain.