Born in Liège, Gillet has played for RFC Liège, Visé, Eupen, Gent, Anderlecht and SC Bastia who Gillet joined on a season-long loan deal with on 10 July 2014.
On 1 November Gillet scored a costly own goal as Bastia lost 1–0 at home to Guingamp.

On December 2015, while at Anderlecht, Gillet said he had turned down an offer from LA Galaxy but that he could still move to Major League Soccer that winter.
He was out of contract at the end of the season.
Eventually on 28 December 2015  has joined FC Nantes from Jupiler League champions Anderlecht.

On 7 August 2017, Gillet signed a two-year contract with Olympiacos, thereby re-uniting with his former manager at Anderlecht, Besnik Hasi.
On 18 November 2017, he scored his first goal with the club opening the score in a 2–1 home Greek Super League game against Levadiakos.
He left Olympiakos at the end of the 2017–18 season.

On 2 August 2018, Gillet signed a two-year contract with French Ligue 2 club RC Lens.
After his second season with Lens, during which he helped the club achieve promotion to Ligue 1, his contract was not renewed.

In July 2020, Gillet returned to Belgium having agreed a one-year contract with Sporting Charleroi.
Born in Liège, Gillet has played for RFC Liège, Visé, Eupen, Gent, Anderlecht and SC Bastia who Gillet joined on a season-long loan deal with on 10 July 2014. On 1 November Gillet scored a costly own goal as Bastia lost 1–0 at home to Guingamp.
On December 2015, while at Anderlecht, Gillet said he had turned down an offer from LA Galaxy but that he could still move to Major League Soccer that winter. He was out of contract at the end of the season. Eventually on 28 December 2015  has joined FC Nantes from Jupiler League champions Anderlecht.
On 7 August 2017, Gillet signed a two-year contract with Olympiacos, thereby re-uniting with his former manager at Anderlecht, Besnik Hasi. On 18 November 2017, he scored his first goal with the club opening the score in a 2–1 home Greek Super League game against Levadiakos. He left Olympiakos at the end of the 2017–18 season.
On 2 August 2018, Gillet signed a two-year contract with French Ligue 2 club RC Lens. After his second season with Lens, during which he helped the club achieve promotion to Ligue 1, his contract was not renewed.
In July 2020, Gillet returned to Belgium having agreed a one-year contract with Sporting Charleroi.
Guillaume Gillet