David studied at Dublin Institute of Technology before moving to Loughborough to train as a full time athlete with coach Nick Dakin in October 2006. Gillick specializes in the 400 metres and won the European Indoor Championship in 2005 and 2007. He set the Irish Indoor record of 45.52 seconds in the 2007 final. The time also beat the Irish outdoor record of 45.58 and was within the Olympic 'A' qualifying standard for the 2008 Games.

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

In the 2005 European Indoor Championships David 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 David to miss the 2005 World outdoor Championships, but the season wasn't a complete write off, David managed to lower his personal best to 45.93

In 2006 David again suffered an injury hit season but did win his first national outdoor 400m 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.

After some soul searching, David made the decision to move to Loughborough in England, to a new training group and coach, Nick Dakin.

Early in 2007, in his first race of the year, David ran a blistering 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 of 45.99, previously held by Belfast's Paul McKee, a record McKee set when winning a bronze medal in the 2003 World Indoor Championships. In March 2007, David successfully defended his European Indoor title, running a new Irish record of 45.52s in the process, again a new Irish record and the qualifying standard for the 2008 Beijing Olympic Games.

David continued his good form into the summer, and had a great World Championships, finishing 6th in his semi-final, and 16th overall. He performed well when it mattered, running both his 2nd and 3rd fastest times ever.