Bitshares I know of, didnt like it, no vol but I havent tried it for a year.
Ripple has a kind of decentralised market, I havent looked at Ripple for a few years so I dont even know what form it takes now.
Bitshares has ok volume for USD and CNY pegged assets, but not for a gold backed one.
The problem with Ripple is that the value of anything (except XRP itself) is at the mercy of some company that in most cases has no legal obligation to by that token back even if they sold it as "dollar" or "gold".