Gold futures' margin
The initial and overnight margin requirements are set by the exchanges (who calculate them using the Standard Portfolio of Analysis of Risk, or 'SPAN' system), and positions are market to market according to these at the end of the trading session. To find these margin requirements you will need to consult the website of the exchange on which the contract you are trading is issued (i.e. if you're trading on the London Metal Exchange it's no good looking at the Chicago Mercantile Exchange's margin requirements as a previous answer suggests!). However, for positions entered and exited within the same day, the daytrade margin rate will apply. This is set by your broker rather than the exchange, and can be as little as 10% of the exchange requirement. You can find a useful comparison of different margin types and requirements in the article I have published here: Understanding Margin for Futures Trading.