He began his football journey at a young age, training at the INF Clairefontaine before joining the youth ranks of Rennes in 2014.
[1] Gasnier spent several years at Rennes, where he developed alongside talents like Eduardo Camavinga.
[6] Unfortunately, his season was cut short in November 2023[7] due to a severe knee injury, requiring surgery and an extended rehabilitation period.
[11][12] Gasnier established himself in Pau's attacking rotation during the season,[13] taking advantage of injuries in midfield to gain playing time.
[14] On 1 February 2025, Gasnier signed for Championnat National club Valenciennes on loan until the end of the season.