Gillick first ran for Ireland at the European Athletics U23 Championships in 2003, however it was not a successful international debut for Gillick. At the 2004 IAAF World Indoor Championships Gillick won a bronze medal as part of the Irish 4x400 metres relay team (along with Robert Daly, David McCarthy and Gary Ryan).

In the 2005 European Indoor Championships Gillick produced two outstanding runs to first win his heat and then win the Gold Medal in a personal best time of 46.30, beating the favourite, Spain's David Canal, into second place. It was Ireland's First gold medal in a sprinting event in 75 years. Injury forced Gillick to miss the 2005 World outdoor Championships.

In 2006 Gillick again suffered an injury hit season but did win the Irish national Championship in a time of 45.67. He qualified with ease through the first round at the 2006 European Championships before running poorly in his semi final and being eliminated.

In October 2006, Gillick became a full-time athlete and relocated to Loughborough in the English midlands to link up with English 400m coach Nick Dakin and a group that included a number of top British 400m athletes. Loughborough University has a great tradition in athletics and is viewed by many as one of Europe's top sporting universities.

Early in 2007, Gillick ran a time of 45.91s at an indoor meeting in Dusseldorf, a performance which made him the world leader over 400m indoors and bettered the Irish Record, previously held by Belfast's Paul McKee. In March 2007, he successfully defended his European Indoor title, running a new Irish record of 45.52s in the process, again a new Irish record. On June 9, 2007 he ran 45.23 seconds over 400m in Geneva to set a new Irish outdoor record.

On June 27 2008, he ran 45.12 in Lille France, improving on his own 45.23, setting a new Irish record and brings him from number 7 on the European list to number 1 (as of end June 08)