The Bombay Stock Exchange benchmark Sensex today gained 102 points in opening trade on heavy buying by foreign funds and retail investors, driven by firming trend on other Asian bourses.The 30-share index, Sensex, rose 101.81 points, or 0.58 per cent, to 17,123.66 points with stocks of oil and gas, auto and metal sectors leading the rise. The wide-based National Stock Exchange index Nifty moved up by 31.95 points, or 0.51 per cent, to 5,084 points.