Gaillard came through the French junior single-seater ladder, performing well in French Formula Renault and then Formula 3 before attracting attention from a Formula One constructor. His form in those categories was sufficiently impressive to earn him a drive with the Ensign team, a small British constructor run by Mo Nunn that competed as an independent entry in the Formula One World Championship.
Gaillard made his World Championship Formula One debut on 1 July 1979. Ensign's N179 was not a front-running car, and Gaillard struggled to extract results from it. In five attempts to qualify and race, he failed to make the grid on three occasions, a reflection more of the car's deficit than the driver's ability given the extremely competitive nature of Formula One qualifying in the period. He was subsequently dropped in favour of Marc Surer, a Swiss driver who would go on to a longer Formula One career.
Gaillard returned to Ensign for the 1980 Spanish Grand Prix at Jarama, and this race produced both the highlight and the frustration of his Formula One career. He finished sixth, the last of six classified finishers, five laps down on the winner — but sixth place in that era was worth a championship point. However, the race was subsequently stripped of its World Championship status entirely, becoming a non-championship event in the official record books. The reason was the FISA–FOCA war, the bitter political and commercial dispute between the sport's governing body FISA and the Formula One Constructors Association that erupted in 1980. With the race reclassified, Gaillard's would-be point was annulled along with all other results, leaving him with nothing to show for what had been his best performance in a World Championship entry.
After his Formula One career ended, Gaillard continued racing in other categories. He competed in Formula 2, CanAm, and sports car events, including appearances at the 24 Hours of Le Mans. Following his retirement from active competition he became a racing instructor, passing on the technical knowledge accumulated across his years at the highest levels of single-seater and sports car racing.
Gaillard's career illustrates the precarious position of drivers in small privateers during Formula One's most intensely competitive era. His technical ability was evident from his junior category results, and his sixth-place finish at the 1980 Spanish Grand Prix — however ultimately invalidated — showed he could complete a race distance competitively. The political circumstances that robbed him of that championship point were entirely outside his control and remain a footnote in the broader story of Formula One's most turbulent administrative period.