The BSE benchmark Sensex gained over 100 points in afternoon trade today. The Sensex was trading at 20,164, up by 139 points a short while ago. The gain in the stock market is due to fresh buying by funds and covering-up of pending short positions ahead of monthly expiry in the derivatives segment tomorrow.Meanwhile, Hong Kong's Hang Seng index was up 0.69 per cent, while Japan's Nikkei gained 0.34 per cent, in early trade today. The US Dow Jones Industrial Average ended 0.18 per cent higher in Tuesday's session.