Martin Brodeur, in his 16th straight start, stopped 23 shots through regulation, and Gerber made 28 saves for Ottawa.
Langenbrunner and Karel Rachunek also scored for the Devils, and Patrik Elias had three assists.
Luke Richardson, who came in with only six points this season, tied a career high with two on a goal and assist. Randy Robitaille also scored for the Senators, the Eastern Conference's top team which has dropped five of seven (2-4-1).
One night after Ottawa's potent top line of Daniel Alfredsson, Dany Heatley and Jason Spezza went pointless in a 5-1 home loss to Buffalo, coach John Paddock shook up the trio by putting Alfredsson with Mike Fisher and newly acquired Cory Stillman, and moving up Dean McAmmond.
It paid off when Spezza and McAmmond set up Richardson's goal.
Richardson scored his first goal in 26 games, dating to Dec. 18, when his shot struck Devils forward David Clarkson, took a hop, and caromed past Brodeur 5:25 in.
It was the second this season for Richardson, a defenseman who has 35 NHL goals in 1,395 games over 20 years.
John Madden was stopped on a short-handed breakaway with 30 seconds left in the first period, and Langenbrunner topped him with a short-handed goal in the middle frame.
Langenbrunner blocked Andrej Meszaros' shot at the New Jersey blue line, and Elias raced with the puck. The New Jersey captain loaded up a slap shot at the left circle and ripped a drive past Gerber to tie it at 6:12 on Ottawa's second power play.
New Jersey netted the go-ahead goal 1:52 later, during 4-on-4 play. Just 17 seconds after a Devils power play was thwarted 8 seconds in, Elias sparked another scoring chance.
New Senators defenseman Mike Commodore, playing his second game since Monday's trade with Carolina brought him and Stillman to Ottawa, was hassled by Elias behind the net and coughed up the puck. Zach Parise got it and sent a pass to the slot where a driving Rachunek one-timed a drive by Gerber.
Brodeur, who passed Ed Belfour into third place on the NHL career minutes played list among goalies, temporarily kept the Devils in front by denying Chris Kelly on consecutive attempts with his glove.
Less than 2 minutes later, Robitaille put a shot in the top-right corner, making it 2-2 with 4:31 left in the second. It was Robitaille's 10th of the season and second in two nights.
Gerber got back in the net after dressing as Ray Emery's backup the previous three games. Emery had posted consecutive wins before allowing four goals in the third period of Ottawa's 5-1 home loss to Buffalo on Tuesday.
Gerber gave up seven goals on 47 shots in his previous two starts - both losses.