A native of Huy, Gillet began his basketball development with his hometown club before spending three years at Wallonia's Central Basketball Academy. Turning professional, he made his Belgian League debut with Liege in 2008 and was a part of their Super Cup victory in 2009 and their first-place finish in the regular season standings in 2010. He has switched to Oostende ahead of the 2013-14 season. Gillet has been a member of the Belgian national team in the European Championships at every youth level, including U16 (2007), U18 (2009) and U20 (2010 and 2011). He made his competitive senior debut in August 2012 in a win over Poland, making 19 appearances in total headed into 2013.