Gillis played on the PGA Tour in 2010 after several years on the Nationwide Tour, and having previously competed on the PGA Tour in 2003 and 2005, and the European Tour from 1998 to 2002. He also played in several PGA and Nationwide Tour events during the 1990s. According to his profile, Gillis has played professional tournaments in 26 different countries.
Gillis won his first title on the Nationwide Tour at the 2009 Nationwide Tour Players Cup. He finished 5th on the money list to earn his 2010 PGA Tour card. In 2010, Gillis finished tied for fifth at the Deutsche Bank Championship. He retained his playing rights for the 2011 season, finishing 76th on the money list. He spent 2014 on the Web.com Tour, but a strong performance in the Finals earned him a place on the PGA Tour for 2015.
In between the time of playing on the PGA Tour, and switching to the PGA Tour Champions, Gillis spent a year coaching high school golf. He coached the varsity team at Pontiac Notre Dame Prep. In 2018, Gillis began playing on the PGA Tour Champions. In his first event, the 3M Championship, he shot 67-67-67 to finish in a tie for 3rd.
