A Great Merchant can conduct a "trade mission" which will immediately give you a whole bunch of gold. To conduct a trade mission, first move the merchant to a wealthy and powerful city as far away from your cities as possible. When the merchant reaches the target city you'll be able to conduct the mission; trade missions can ONLY be carried out in foreign cities. The value of the mission increases the further away and wealthier the target city is. Great Merchants can also settle permanently in a city, providing a continuing large commerce and small food boost to the city.