Gastien started his career with his home-town club Niort and made his debut in the 2–1 victory over Libourne-Saint-Seurin in Ligue 2 on 28 March 2008, coming on as a substitute for Ronan Biger.[2] On 27 March 2010, he scored his first senior goal in Niort's 1–0 victory over Toulouse Fontaines, but was sent off later in the match.[3] Gastien was part of the team that won promotion from the Championnat de France amateur in 2010 and subsequently promotion back to Ligue 2 two years later. In total, he played 140 league matches during six senior seasons with the club, scoring 14 goals.[citation needed]
On 25 May 2013, it was announced that Gastien had agreed a contract with Dijon for the 2013–14 season.[4]
Personal life
He is the son of football manager and former player, Pascal Gastien.[4] Johan has been coached by his father at Niort and Clermont.