Stock markets opened on a slightly better note on the last trading day of 2012 with the BSE benchmark Sensex gaining over 27 points in early trade. The 30-share index rose by 27 points to trade higher at 19,472 points with stocks of oil and gas, auto and power sectors rising.The broad-based National Stock Exchange index Nifty rose by 4 points to 5,913.Brokers said, selective buying in index-related stocks by participants mainly helped Sensex to trade in the positive territory.