At the age of six years, the son of a doctor and an insurance businessman playing tennis began. He turned professional in 2002. 2004 Simon was in Metz his debut at an ATP tournament. 2005, the Frenchman reached the quarterfinals of Casablanca and stood for the first time at the French Open at a Grand Slam tournament in the main draw, where he defeated his compatriot Olivier Patience in the first round.
2006 Simon reached after beating Nicolás Massú and Tomáš Berdych the third round of the Australian Open, where he lost to Thomas Johansson. In April, he reached the final of the Tournament of Valencia, but was defeated Nicolás Almagro from Spain.
2007 Simon won in Marseille for the first time an ATP tournament. On 19 March 2007, the Frenchman was ranked 38 in the world rankings, his highest placing up to that point. In the same year he reached the second round at the French Open and Wimbledon.
Simon 2008 managed the big breakthrough. When Masters in Cincinnati he beat the former world tennis rankings - First Roger Federer. In the semifinals, he had to admit defeat Nicolas Kiefer. At the 2008 U.S. Open he lost in the third round of the Argentine Juan Martín del Potro. After he defeated the Spanish " oldie " Carlos Moyá final in Bucharest for the first time Simon rose to 13th in the world rankings.
In 2009 he started as number six of the seedings at the Australian Open and reached the quarterfinals where he retired against eventual tournament winner Rafael Nadal 2:6, 5:7, 5:7. The French Open went for him after a 4:6, 4:6, 2:6 against the Romanians Victor Hanescu in the third round to end. At Wimbledon, he defeated Victor Hanescu and reached the second round where he was defeated by Juan Carlos Ferrero reached via wildcard into the main draw.
After he had failed at the U.S. Open in 2010 in the third round at the world number one Rafael Nadal, Simon won in September 2010 in Metz his seventh ATP title.
The year 2011 began for Gilles Simon with his eighth ATP title in Sydney. At the Australian Open, he retired in the second round against Roger Federer 2:6, 3:6, 6:4, 6:4, 3:6, and thus suffered their first defeat against the Swiss in the third encounter. This was followed in Hamburg the ninth ATP title and at the same time with the first ATP World Tour 500 rankings.