Buying gold is not sustainable solution to US dollar reserves since gold is a limited quantity. If China keeps buying gold, value of gold will sky rocket but that's just speculative value of gold. Eventually the value of gold might fall causing losses for China.
What other option does China have if they want sanction proof their reserves? Buying Euro, Yen or any pro-western country is also not an option I think. Cause all of these countries could sanction China just like they sanctioned Russia.
Any other option?