The Sensex at the Bombay Stock Exchange gained 40 points, or 0.2 percent, to close at 20,122, in a special trading session today. Trading was held between 11:15 a.m. and 1:15 p.m. to test its disaster recovery software. The Nifty at the National Stock Exchange rose 13 points, or 0.2 percent, to close at 6,107.