Sensex at the Bombay Stock Exchange gained 120 points, or 0.6 percent, to 20,509, on the last day of the year on Friday. Climbing for the third straight session the market rose on sustained buying by investors amid indications that strong economic growth will boost company earnings. The Nifty at the National Stock Exchange fell 33 points, or 0.5 percent, to 6,135.Stock markets in China, Hong Kong and Singapore ended mixed.