The benchmark BSE Sensex gained 158 points to 21,017 in early trade today on sustained buying by funds and retailers amid a firming trend on other Asian bourses. The National Stock Exchange index Nifty moved up by 47 points to 6,264.Among other Asian markets, Hong Kong's Hang Seng index rose by 1.13 per cent, while Japan's Nikkei gained 1.60 per cent in early trade today. The US Dow Jones Industrial Average ended 1.84 per cent higher in yesterday's trade.