Bullion is stored in a bonded storage facility and every single deposit is itemised and to tagged to their legal owner. The bonded storage facility is a tax free zone.
Any product of which GST is payable may have its GST payment suspended (for the time being) if it is stored in a Zero-GST warehouse, such as Le Freeport Singapore. If and when the product is removed from the Zero-GST warehouse, GST becomes immediately payable.
You may read more about the GST: Guide on Exemption of Investment Precious Metals (IPM) by the Inland Revenue Authority of Singapore for details.