Marseille, who finished 10th last season, have made a perfect start with 12 points from four games, two ahead of second-placed Olympique Lyon.
The 2010 champions, whose fans greeted new signing Joey Barton by displaying a huge English flag, went ahead in the 35th minute when full-back Jeremy Morel shot past goalkeeper Benoit Costil after a fine move initiated by Gignac on the right.
Marseille were in control until Rennes, who had barely threatened their hosts for an hour, levelled when Julien Feret chested down from a cross to fire the ball home in the 57th minute.
However, Gignac, who narrowly missed the target from close range three minutes after the equalizer, eventually gave his side an new edge when he superbly half-volleyed in from 25 metres. Romain Danze's own-goal completed Marseille victory.
Gignac, who only scored one goal in Ligue 1 last season, secured Marseille's win at Montpellier last weekend.
At Lille, Zlatan Ibrahimovic scored twice in the first half to bring his personal Paris Saint-Germain tally to four goals from three games.
The former AC Milan front-man struck after just 27 seconds when he netted from close range after a lightning move down the left side.
Lille, widely regarded as PSG's main rivals in the title race this season, immediately upped the pressure, with their hard work paying off when centre-back Aurelien Chedjou headed home on 12 minutes from a Dimitri Payet corner.
Ibrahimovic triggered a great one-two with playmaker Javier Pastore in the 21st minute, to then run through on goal and shot past keeper Mickael Landreau.
The Swede seemed in determined mood and made a point of showing there was more to his game than scoring goals, marshalling the team throughout the game, doing his share of defensive tasks and shouting out advice.
His PSG team-mates duly responded with their best performance so far this season.
Lille had the better of the possession throughout the second half, but were never able to truly test Salvatore Sirigu in the visitors' goal, and now lie 11th in the table on five points.
Earlier, Mahamane Traore's stoppage-time goal deprived 10-man Girondins Bordeaux of a home win, earning Nice a 1-1 draw.
Bordeaux, a man down after Ludovic Sane was sent off on 63 minutes for slapping striker Dario Cvitanich, struggled to create chances throughout until Henri Saivet's header gave them the lead in the 86th minute.
But Nice equalised six minutes later when Traore superbly lobbed goalkeeper Cedric Carrasso who was rooted on the penalty spot.
Unbeaten Bordeaux are fourth on eight points from four games. Nice are 17th on three.
