Strange but untrue actually. See my other post on this.
Yes, gold shows up in the FX lookup table, but so does the Zimbabwe Dollar, Afghanistan Afghani and Tonga Pa'anga - and I wouldn't expect Singapore Airlines to accept those either. Silver is also listed - basically anything with a valid currency code will show up, but only for reference, not as a form of payment.
For the avoidance of all doubt, Singapore's lookup page is here