The BSE benchmark Sensex gained 77 points in opening trade today, extending Friday's gains on continued buying by foreign funds amid a firming trend overseas. The 30-share index rose by 77 points, or 0.47%, to 16,310 on the back of buying in IT, refinery, consumer durable and tech sector stocks. The index had gained 440 points, or 2.79%, in the previous session.In a similar fashion, the wide-based National Stock Exchange index Nifty moved up by 16 points, or 0.33% to 4,904.