Each gold holding (maybe bar, maybe vault) is itself centralized.
I would consider a promise from pg or sama to pay 1oz of gold upon presentation to be pretty low on counterparty risk; I'd consider the same from a random person online who claimed to have gold and be willing to redeem it upon presentation of one of his tokens as a much higher counterparty risk. The market would price these separate issues, and then you could build a basket of gold issues into something meta (maybe with a blockchain, or maybe with smart contracts, to reduce the meta counterparty risky).
I am a bit confused. My impression so far was that something like this would be commonly seen as a decentralized scenario?