You can invest at most 10% of CPF OA investible amount into Gold ETF (O87). 1 lot 10 shares is about 1660 USD now - buy online using local online brokerage adn select CPF as payment method. Of course you will need to have a brokerage already, as well as CPFIS Account with UOB, OCBC and DBS. Suppossedly you may be able to use CPF for UOB gold savings account, but i havent been able to verify that.
Speculating purely in gold is risky, you need to decide when to enter as well as exit. You may wish to invest CPF more safely using a passive diversified portfolio strategy containing stocks, gold, bonds and cash.