Erasing early gains, the Bombay Stock Exchange benchmark Sensex fell nearly 109 points today. The Sensex, which had climbed to 17,035 in the morning trade, gaining 187 points, fell back to close at 16,730 points.The broad-based National Stock Exchange index Nifty lost 37 points to 5,035, after climbing to 5,132 as stocks in metals, realty, bank and oil and gas sector fell.Global markets were down after Germany showed stagnant growth numbers, knocking down European stocks and fuelling concerns about the health of the world economy. Asian stock markets, barring Tokyo, also closed lower.