Arsenal climbed to second with a 2-1 win at Everton before Sunderland produced an inspired display at the home of the champions to complete one of the upsets of the season.
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Onuoha's goal, the first by an opposition player in the league at Chelsea this season, came just before half-time following a mazy dribble Lionel Messi would have been proud of which left three Chelsea defenders trailing in his wake.
Gyan's strike completed a brilliant build-up following swift passing from Jordan Henderson and Welbeck, leaving the club's record signing free to plant the ball perfectly past Petr Cech.
Their lead at the top was cut from three to two points by Arsenal's win at Everton. Manchester United, who drew 2-2 at Aston Villa on Saturday, are a point behind Arsenal in third on 25 points.
"Two weeks ago we were hung, drawn and quartered (by Newcastle) and we wanted a response," Sunderland manager Steve Bruce said.
"We're looking at how well we played and we thoroughly deserved to win today. Chelsea are a great team, but all-round I'm delighted for the boys because they worked hard to get over the Newcastle performance.
"Two weeks ago was the worst, today... well, I'm not going to come to Chelsea and win 3-0 very often. We'll enjoy a few glasses tonight and take it easy."
"Sunderland played with a fantastic spirit and they played better," Chelsea manager Carlo Ancelotti told Sky Sports.
"We were not able to come back into the game after going behind and it was a bad day.
"We were not able to play our football for 90 min
