The Gold Guarantee is (was?) a company based in Singapore allowing "investments" in gold. They had two separate schemes. One allowed victims to take delivery of their gold, but at an approximately 30% premium, albeit with a monthly repayment of a small portion of the premium for as long as the gold was held (and the company remained solvent) and an option for the company to buy back the gold at a price related to the spot price. The other scheme offered a gold certificate and a higher monthly payout.
The monthly payouts amounted to over 20% p.a. I am unaware of any method by which a company could sustain such payouts through normal business practices. Were I aware of such a business, I would invest in it.