It's basically an ETF. You buy and sell and it tracks the international gold spot rates. The spread between buy and sell is fairly small so quite good value. There are no storage costs or anything like that - the bank makes money from the spread on transactions.
I would be careful though - unlike currency deposits these ETF funds are not guaranteed should HSBC go bankrupt. If HSBC goes out of business you will lose your gold holdings entirely. I have therefore recently sold up all my 'paper' gold investments.
If you're holding gold as a hedge against incoming major financial shock that might bankrupt most financial institutions then probably best to hold physical gold - can buy coins and bars over the counter at Hang Seng.