The Sensex at the Bombay Stock Exchange gained 75 points, or 0.4 percent, to close above the 20,000 level for the first time in two years, at 20,039, on buying especially in oil stocks, some good corporate results, and firm global markets, today. The Nifty at the National Stock Exchange rose 25 points, or 0.4 percent, to 6,064.Stock markets in Japan, China, Hong Kong, South Korea and Singapore climbed between 0.5 percent and 3 percent.