Gignac, who joined the club in 2010 for around 20 million euros, has already scored twice this season to help Marseille to the top of Ligue 1 with nine points from three games.
"I always wanted to give my best but I could not as I never was completely fit," Gignac, who was the Ligue 1 top scorer with 24 goals in 2008-09, told the club website ahead of Marseille's game against Stade Rennes on Sunday.
"I work a lot and I am getting close to my top form.
"I owe the club a lot, the fans who had were been disappointed with me. But this is over. I am back with a vengeance."