Gillies was the second overall pick by the Saskatoon Blades in the 2004 Western Hockey League (WHL) Bantam Draft. Gillies spent parts of four seasons with the Blades, but the team struggled and consequently Gillies played in only 10 playoff games during his major junior career. Still, he gained attention with his strong physical and two-way play, and was selected to represent Canada at the 2007 IIHF World U18 Championships in Finland, where Canada finished 4th overall.