If there are thought to be some traders who would have forwarning of an impending attack, they'd be trying to buy up gold (since all paper currencies would be risky with a major war or attack going on - nobody knowing what to expect, and with the possibility of a major economic meltdown). That would drive the price of gold up.
If the price of gold stayed the same or went down, that might not indicate that there was no attack - just that there weren't a significant number of traders who had foreknowledge of it. If the price dropped significantly, it might indicate that some traders had inside knowledge that an attack had been foiled.... again not an infallible indication.