Jimmy Gressier first competed in international championships in 2015. In March he started as part of the U20 race at the Cross Country World Championships in Guiyang, where he finished 80th. Things went better at the European Cross Country Championships in his home town of Hyères, where he finished fourth in the junior race. A year later he took the same place in the same race at the European Cross Country Championships in Chia. In June he was the French runner-up U20 champion over 1500 meters in 3: 55.82 minutes. A month later, he started 5000 meters at the U20 World Championships in Bydgoszcz and finished tenth in the final. In 2017 he also won the silver medal in the U23 age group at the national championships, although he ran seven seconds faster than a year earlier. In the summer he was disqualified over 5000 meters at the U23 European Championships , which, like the year before, the Junior World Championships took place in Bydgoszcz. At the European Cross Country Championships in Šamorín, however, he was able to win the gold medal in the U23 age group.
In June 2018, Gressier competed in both the U23 Mediterranean Games in Jesolo and the regular Mediterranean Games in Tarragona, Spain. While he was victorious over 5000 meters in the U23 age group, he finished eighth in the final in Tarragona in 14: 01.13 minutes. At the European Cross Country Championships in Tilburg, he repeated his 2017 triumph. In February 2019, he became French vice indoor champion over 3000 meters. A few weeks later, he started at the European Indoor Championships in Glasgow over this distance . He moved into the final, in which he finished seventh in 8: 00.89 minutes. In summer he started the 5000 and 10,000 meters at the U23 European Championships in Gävle. After setting a new 5000-meter best time in June in 13: 23.04 min, in the final 14: 16.55 min was enough for him to win the gold medal. Two days earlier he won the gold medal over twice the distance in a best time of 28: 44.17 minutes. In December Gressier won gold for the third time at the European Cross Country Championships in the U23 age group, this time in Lisbon.
In February 2020, Gressier finished second in a 5-kilometer road race in Monaco in 13:18 minutes. This time marked a new European record, previously held by the Swiss-French athlete Julien Wanders . In August he improved his best time over 5000 meters at the Diamond League Meeting in Monaco to 13: 15.77 minutes.