The French coach was appointed as manager at the Stade de la Mosson outfit for the second time this week, but could not end their now nine-match winless Ligue 1 run at the first time of asking.
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Tabanou tried his luck with his weaker foot a short while later, cutting inside from the left to unleash a 25-yard shot which may have had Jourdren beaten, but flew past the post.
It took until the 60th minute for Montpellier to show that they were also a goalscoring threat as an intended cross from Souleymane Camara's forced Stephane Ruffier into a reflex stop at his near post.
The hosts came even closer two minutes later when Saint-Etienne captain Loic Perrin nearly turned the ball into his own net, slicing a clearance past Ruffier and only a few inches wide of the goal.
Just as it appeared as though Montpellier were mounting some pressure, the visitors netted a 75th-minute winner as former Leeds United striker Gradel converted a Faouzi Ghoulam cross from close range.
The result lifts Saint-Etienne two places to sixth in the Ligue 1 table, while Montpellier are in 17th position, five points above the relegation zone and without a win since October 6.