The Bombay Stocks Exchange benchmark Sensex gained over 166 points in opening trade today. Later it lost its initial gains and was trading almost flat at 18,632 a short while ago. . Brokers said fresh buying by funds and retail investors amid a firming trend in Asia mainly boosted the trading sentiment. Meanwhile, Japan's Nikkei index was up by 0.22 per cent and Hong Kong's Hang Seng Index by 0.51 per cent in early trade today.