Brazilian striker Marco had given the visitors the lead in the 71st minute as they sought would have been just a fourth win of the season.
However, a Florian Jarjat own goal levelled the scores with nine minutes left and, buoyed by the equaliser, Gignac struck a minute later to seal the points and take advantage of PSG’s defeat against Reims to move within five points of the summit.
The home side started brightly and tested Yoann Thuram after five just minutes, the Troyes goalkeeper deflecting Mathieu Valbuena’s effort away from danger.
Valbuena turned provider as he crossed for Andre Ayew after 19 minutes, but his header did not find the target.
His brother Jordan volleyed straight at Thuram, before Valbuena forced another decent save from the stopper.
Steve Mandanda was finally tested after 62 minutes to keep out Benjamin Nivet’s effort, before Marcos shot just wide six minutes later.
Troyes were on top and took advantage of their period of domination when Marcos dispossessed Nicolas Nkoulou before racing in on goal and firing past Mandanda.
The goal woke up OM, who struck back nine minutes from time when Nkoulou’s header deflected off Jarjat and past a wrong-footed Thuram.
A minute later and they were ahead. Substitute Modou Sougou got free inside the penalty area on the right, and he passed to Gignac who finished from close range.
Elsewhere Sunday, Lyon were unable to pick up the win they needed to move level with PSG after drawing 1-1 at Brest on Sunday.
Kamel Chafni gave the strugglers an eighth-minute lead, only for Tripy Makonda’s own goal eight minutes into the second half to earn Remi Garde’s visitors a point.
The draw means Lyon trail PSG by two points while Brest move five points clear of the bottom three.
Lille battled from a goal down to maintain their European push with a hard-fought 2-1 victory over Bordeaux at the Grand Stade Lille Metropole this evening.
Les Girondins headed into the game looking to arrest a three-game losing run, and took the lead through a Jaroslav Plasil penalty just after the half-hour mark.
But Rudi Garcia’s side responded well and turned the game on its head thanks to second-half goals from Ronny Rodelin and Salomon Kalou to secure their fourth straight league win and climb to seventh in the standings.