The striker netted his 10th goal in nine outings as the hosts took the lead early on. Gignac capitalised on Valenciennes defender Eloge Yamissi’s weak back pass to round goalkeeper Nicolas Penneteau and stroke the ball home.
Marseille continued to dominate and went in search of their second goal but were pegged back just 15 minutes after the opener as defender Arthur Masuaku grabbed an equaliser.
"Everyone was committed tonight, our team spirit was irreproachable," Gignac said. "We're six points behind (third-place) Lille now. I hope we carry on like this because we have two home games coming up."