Career
Gastien started his career with his home-town club Chamois Niortais 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. 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. In total, he played 140 league matches during six senior seasons with the club, scoring 14 goals.
On 25 May 2013, it was announced that Gastien had agreed a contract with Dijon for the 2013-2014 season.
He is the son of former football player and former manager of Chamois Niortais, Pascal Gastien.