The Sensex at the Bombay Stock Exchange gained 115 points, or 0.6 percent, to close at 20,535, today, amid the expiry of the monthly derivative contracts, and rising global markets. The Nifty at the National Stock Exchange rose 35 points, or 0.6 percent, to 6,092.Stock markets in Japan, China, South Korea and Singapore climbed between 0.5 percent and 1.8 percent.