If u see the Sources to invest the fund for the investors are equity and Gold
When market goes up, Investors will be interested in putting the money in equity, hence pull out the money from the gold source and hence demand decreases, no buyers and hence the gold price falls.
in the otherway round, when market falls, investors will be interested in putting the money in gold from equity and hence the share market fall because of less buyers...
Normally when sensex is going upside, Gold prices are going towards downside beacause the investors are withdrawing money from one low earning stock exchange to high revenue stock exchange. So, sensex & Gold prices are related in inverse order.