If China sells USD to buy gold, then gold price vs the dollar would go up, that however doen't mean RMB will go down with the USD. If RMB is gold backed instead of pegged on dollar, it only means RMB will go up along with gold. At this stage it's not in China's interest to see the RMB go up against the dollar. However as long as it has a large trade surplus, it will have the USD to slowly accumilate gold.
........No........Because free market follow supply and demand.
If China sell its 2 trillions USD Bond along with any USD Forex, then it will place a sell order in the market, which means the supply of USD Spike by whatever the Sell Order entail, assuming no demand increase (such as an
INCREASE of demand of USD to pay bill by a third country, say Japan) the Demand of USD remains the same. Then according to the basic supply and demand rules, USD devalue. And the value of RMB get back in each subsequent Sell order will be lower.
On the other hand, if China buy golds, which they will place in a buy order, unless China buy gold using USD (Which will then negate the RMB backed gold system), the demand of Gold increase and the supply remain the same. And then the gold price would spike. Worse case scenario. China buy gold with RMB but reference to the price of USD as per international gold market. Then on one hand, RMB will lose value and inflate (by the act of selling USD) and gold will also be inflated (as USD is weaken)
For example. A gold worth 1245 USD per tray ounce today. And 1 USD to 6.4 RMB.
If China start selling USD, then the USD will be weaken, and RMB will be strength, So, 1 USD will worth less (weaken) than 6.4 RMB, let's say it's now 5 RMB to 1 USD. Then if the gold price remain unchanged. Then to buy 1 once of gold, China would have to pay 1,743 RMB/ounce more than before selling USD
And finally, if China have a gold backed RMB, then they would have enough gold to back the RMB currency structure already, they would not be buying gold. The problem is, how do you get enough gold to back your currency structure?
Also, do remember, 40-45% of world gold are privately own, even if China sourced about 55% of the other gold, Chinese economic is still too big to back by gold, US have about 60-70% of world gold reserve in the 1950s yet the Bretton Wood system still fail, go figure.