Gillet was born in Macksville, New South Wales and moved with his family to Bribie Island, Queensland at the age of three years.
He played his junior football for the Bribie Island Warrigals and Morayfield State High School. In 2008 Gillett started the year with the West Panthers club, then halfway through the year Gillett joined the Brisbane Broncos under-20s team where he played several games. In 2009 Gillett played for the Norths Devils in the Queensland Cup. He was named as the Queensland Cup rookie of the year and also played for City in the Queensland Cup City vs Country match.
After Gillett's impressive year in the Queensland Cup, Brisbane Broncos head coach Ivan Henjak rewarded Gillett with a preseason in 2010. Gillett made his NRL debut for the Brisbane Broncos in round 1 2010 NRL season opener against the North Queensland Cowboys off the interchange bench in which Gillett scored a try on debut in the Broncos 30-24 win at Suncorp Stadium. On 23 June 2010, Gillett extended his contract with the Brisbane Broncos for a further three-years, keeping him at the NRL club until 2013. On 8 September 2010, Gillett won the 2010 Rookie of the Year Award at the 2010 Dally M Awards presentations. Gillett finished his NRL debut year with him playing in 21 matches and scoring 12 tries. Gillett was named the 2010 Brisbane Broncos season's rookie of the year. In September 2010, Gillett was selected for the Prime Minister's XIII and played in the 30-18 win over Papua New Guinea.
Gillett finished the Broncos 2011 NRL season with him playing in 19 matches and 6 scoring tries. In the preliminary finals match against the Manly-Warringah Sea Eagles at the SFS, Gillett filled in at five-eighth for the retiring Broncos legend Darren Lockyer after Lockyer suffered a cheekbone injury in the 13-12 Semi Finals win against the St George Illawarra Dragons at Suncorp Stadium.
Gillett was selected as the 18th man in the Australian Kangaroos for the 2012 Anzac test in April. However, he didn't make his test debut. A month after the ANZAC test, Gillett was selected for Queensland Maroons and played in all 3 games of the 2012 State of Origin series as Queensland won the 2013 Origin Series; their 7th in-a-row. Gillett finished off the Broncos 2012 NRL season with him playing in 23 matches and scoring 6 tries.
In April 2013, Gillett re-signed with the Brisbane Broncos until the end of 2016 NRL season. Gillett played from the interchange bench in all three games of the 2013 State of Origin series, helping Queensland extend their record for consecutive series victories to eight. Gillett finished off the Broncos 2013 NRL season with him playing in 22 matches and scoring 7 tries. In 2013 Gillett got selected for the Prime Minister's XIII and played in the 50-6 win over Papua New Guinea.
On 14 February 2014, Gillett was selected in the Broncos inaugural 2014 Auckland Nines squad. In 2014, Gillett was selected for Australia to make his test début against New Zealand in the 2014 Anzac test. Gillett was originally named the 18th man but he replaced club teammate Sam Thaiday to make the 17 man team. Gillett was selected for the Queensland Maroons in played in all 3 matches of the 2014 State of Origin series with New South Wales winning their first origin series since 2005. In round 21 against the Manly-Warringah Sea Eagles, Gillett played his 100th NRL career match for the Broncos in the Broncos 16-4 loss at Brookvale Oval. Gillett finished off the Broncos 2014 NRL season with him playing in 21 matches and scoring 10 tries. On September 2014, Gillett was selected for the Australia Kangaroos Four Nations train-on squad but withdrew due to injury.
In round 1 of the 2015 NRL season, against the South Sydney Rabbitohs, Gillett scored the Broncos first try off the season off the interchange bench in the Broncos 36-6 loss at Suncorp Stadium. Gillett was selected in all 3 matches for Queensland in the 2-1 2015 State of Origin series win, scoring 2 tries. On 4 October 2015, in the Broncos Queensland derby 2015 NRL Grand Final match against the North Queensland Cowboys, Gillett played at second-row in the Broncos historic golden point 17-16 loss. Gillett finished the season having played in 25 matches and scoring 3 tries for the Broncos.
On 6 May 2016, Gillett played for Australia against New Zealand, starting at second-row in the 16-0 win at Hunter Stadium. For the 2016 State of Origin series, Gillett played for Queensland, starting at second-row in all 3 matches in the Maroons 2-1 series win. During the season, Rumours were floating around about where would Gillett will end up after season’s end due to his contract expiring, Gillett reportedly had talks with the Cronulla-Sutherland Sharks, the Gold Coast Titans, the Manly-Warringah Sea Eagles and the New Zealand Warriors. In round 26, against the Sydney Roosters, Gillett played his 150th NRL match in the Broncos 24-14 win at Suncorp Stadium. On 13 September 2016, Gillett agreed to re-sign with the Broncos on a 4-year extension, to the end of the 2020 season after months of speculation of who he would sign with. On 28 September 2016, at the 2016 Dally M Awards, Gillett was named as the Dally M Second-rower of the Year after another outstanding season for the Broncos. Gillett finished the 2016 NRL season with him playing in 21 matches and scoring 3 tries for the Broncos. On 20 September 2016, Gillett was added to the Kangaroos 2016 Four Nations train-on squad. On 4 October 2016, Gillett was selected in the Australian Kangaroos final 24-man squad for the tournament. On 13 November 2016, when the Kangaroos played against England, Gillett scored his first international try for Australia in the 36-18 win at London Olympic Stadium. Gillett played in 4 matches and scored 1 try of tournament including starting at second-row in the Kangaroos 34-8 Four Nations Final win against New Zealand at Anfield.
On 5 May 2017, Gillett played for Australia in the 2017 ANZAC Test against New Zealand where he started at second-row and was awarded as the Man of the Match after toppling the tackle count with 46 during the 30-12 win at Canberra Stadium.