[citation needed] Gastien's career started when he signed his first professional contract as an 18-year-old with then Division 2 side Angoulême.

However, he found it difficult to break into the first team set up, playing only six league games, and left after just one season as a professional.

[citation needed] In order to play more football, Gastien returned to Niort, who had since been relegated back to Division 2, in the summer of 1989.

[citation needed] Gastien went on to play for Nice for three-and-a-half seasons, but again found first-team chances in the top-flight limited, making only 61 league appearances in this time.

[citation needed] Gastien played a major role in the 1993–94 season as Châteauroux finished top of the National, achieving promotion to Division 2.

[citation needed] In October 1999, Gastien was appointed joint caretaker manager of Niort alongside former Rennes boss René Cédolin, but the pair did not take charge of any league games.

[citation needed] In the 2010–11 campaign, Gastien led the club to an eleventh place in the third tier; in the 2011 close season, several new players including Jimmy Roye and Julien Ricaud were recruited and the following season, Niort went on to finish as runners-up to Nîmes Olympique thanks to a 1–0 win away at Gazélec Ajaccio on the final matchday.

[10][11][12] On 4 June 2014, Gastien was appointed as the new manager of his former club as a player, Châteauroux, succeeding Jean-Louis Garcia.