Second-placed Bordeaux, who have not won the league title since 1999, have 41 points from 21 games and trail Lyon by a point. Lyon beat Grenoble 2-0 earlier on Saturday with goals by midfielders Ederson and Cesar Delgado (pictured).
Gourcuff, who drew comparisons with Zinedine Zidane after scoring a superb goal against Paris St Germain last weekend, headed home in the 11th minute as Bordeaux grabbed their fifth consecutive league win.
Morocco striker Marouane Chamakh headed the second on 70 minutes as Nantes were pushing hard. The hosts scored a consolation goal through Filip Djordjevic with two minutes remaining.
"It's still a very long way to go, there are 17 more matches," Bordeaux striker Fernando Cavenaghi told TV channel Orange Sport.
Lyon went ahead against Grenoble in the 14th minute when Brazilian midfielder Ederson collected a Karim Benzema cross in the box before firing past Gregory Wimbee.
Grenoble never gave up at a packed Stade des Alpes but Lyon sealed the win 13 minutes from time when Argentine midfielder Delgado latched on to a Kim Kallstrom cross from the left.
Olympique Marseille are third, four points off the pace, after goals by Mathieu Valbuena and Ronald Zubar earned a 2-0 victory over bottom club Le Havre.
Toulouse are fifth on 37 points after a 3-0 thrashing of Nancy.
St Etienne, who drew 1-1 at home to Le Mans, are three points clear of the relegation zone, with third from bottom Sochaux having a game in hand at Paris St Germain on Sunday.
Also on Sunday, fourth-placed Stade Rennes look to extend their unbeaten streak to 19 matches when they travel to Lille.
