Born and brought up in Halifax, Yorkshire, Widdop played his junior rugby for King Cross before moving to Melbourne, Australia with his family at the age of 16 where he attended Bayside College. As a teenager Widdop went to the 2006 NRL Grand Final and continued to play in the Melbourne Rugby League for Altona Roosters under his father Gary. He was introduced to the Storm by Dean Lance and was taken into the Storms Academy U18 program by Paul Bramley the High Performance Manager at the time, he went on to represent Victoria and the Australian Affiliated U18 Schoolboys and he eventually signed for Melbourne Storm's Under 20s team. On 2 September 2009, Widdop was named on the interchange bench in the 2009 NYC Team of the Year. In the 2009 NYC Grand Final against the Wests Tigers NYC Team, Widdop scored the match winning try and goal in the 24-22 win. Widdop was named the Storms' NYC player of the year in 2009.
In 2010, Widdop's first season in the first team at the Melbourne Storm, he was named in Melbournes' 23-man touring party in preparation for the 2010 World Club Challenge against Leeds Rhinos but was not chosen to play but did pick up the man of the match award in their warm up game against the Harlequins. His first game for the Storm was in Round 7 against the New Zealand Warriors where he played centre in the Storms' 40-6 win at Etihad Stadium. In Round 12 against the Canterbury-Bankstown Bulldogs, Widdop made his first appearance at fullback in place of the rested Billy Slater, scoring a try and kicking 3 conversions in the 23-12 win at AAMI Park. Widdop was put back in the centres for his third and final appearance in 2010 for the Storm, the Round 25 clash against the Wests Tigers.
On 1 June 2010 Widdop received a surprise call-up into new England coach Steve McNamara's 19-man squad for the match against France on 12 June 2010. Widdop featured at fullback and scored a try with his first touch of the ball, he also kicked 2 goals when Kevin Sinfield was substituted in England's 60-6 win. On 9 September 2010, Widdop was named at five-eighth in the 2010 New South Wales Cup Team of the Year. Widdop finished his debut year in the NRL with him playing in 3 matches, scoring 1 try and kicking 3 goals for the Melbourne Storm in the 2010 NRL season. In mid-September, Widdop was once again called up to the England squad for the Four Nations, he started at fullback in an 18-all draw in the warm up match against New Zealand Māori in which he kicked 3 goals at Mt Smart Stadium. Widdop was once again on the scoresheet as England lost their opening game 24-10 against reigning world champions New Zealand, scoring a try and kicking a goal in the 24-10 loss at Westpac Stadium. Widdop was not selected for the game against Australia but was on the interchange bench in the final game against Papua New Guinea where he came on at centre to kick a goal in the 36-10 victory at Eden Park.
On 14 May 2011, Widdop re-signed with the Storm until the end of the 2013 season. Widdop appeared in 25 matches, scoring 2 tries, kicking 7 goals and kicking 1 field goal for the Melbourne Storm in the 2011 NRL season. In the opening match of the 2011 Four Nations, Widdop was named on the interchange bench for England against Wales where he scored a try with the final play of the game in the 42-4 win at Leigh Sports Village. Widdop featured in England's final match against Australia on the interchange bench in a 30-8 loss at Elland Road. Widdop played in all 4 matches and scored a try in the tournament.
On 30 September 2012, in the Storm's 2012 NRL Grand Final match against the Canterbury-Bankstown Bulldogs, Widdop played at five-eighth in the Storm's 14-4 victory. Widdop finished the 2012 NRL season for the Storm playing in 25 matches, scoring 4 tries, and kicking 12 goals.
Widdop started the 2013 season as five-eighth in the Melbourne Storm's 18-14 win over Super League champions Leeds Rhinos in the 2013 World Club Challenge at Headingley Stadium. On 19 April 2013, it was announced that Widdop had signed a four-year contract with the St. George Illawarra Dragons starting in 2014. In Round 15 against Gold Coast Titans, Widdop suffered a dislocated hip in an awkward tackle in the Storms' 18-12 loss at Cbus Super Stadium. Widdop later returned in the Week 1 finals match against the South Sydney Rabbitohs in the Storms' 20-10 loss at ANZ Stadium. Widdop finished his last year with the Melbourne Storm with him playing in 16 matches, scoring 3 tries and kicking 3 goals in the 2013 NRL season. Widdop was selected to represent England from the interchange bench against Italy where he came on and kicked a goal in the 15-14 loss. Widdop featured in 3 matches and kicked a goal for England in the 2013 Rugby League World Cup.
In Round 1 of the 2014 NRL season, Widdop made his club debut for the St George Illawarra Dragons against the Wests Tigers, Widdop proved his worth having a hand in four of the team's seven tries and kicking eight from eight goals in the Dragons 44-24 win at ANZ Stadium. The signing of Benji Marshall to the Dragons part way through the season meant a new halves partner for Widdop. In Round 16, against his former club the[Melbourne Storm, Widdop scored his first club try for the Dragons in the 24-12 win at WIN Stadium. Widdop finished off his first year with the St George Illawarra Dragons as the clubs highest point scorer with 137 points from 3 tries, 61 goals and 3 field goals, and played in all of the Dragons 24 matches. Widdop was selected in England's 2014 Four Nations squad. Widdop featured against Samoa, Australia and New Zealand in the group stages. Widdop scored in all three matches with six, two and one goals respectively.
Widdop featured in the World Club Series against Warrington Wolves where he kicked 3 goals in the 18-12 win at Halliwell Jones Stadium. In Round 6, against the Canterbury-Bankstown Bulldogs, Widdop played his 100th NRL match in the Dragons 31-6 win at ANZ Stadium. In the Dragons week 1 elimination final match against the Canterbury-Bankstown Bulldogs, Widdop became the hero and the villain in the tight match, Widdop was the hero in the 80th minute when he kicked a penalty goal after Bulldogs second-rower Shaun Lane scored a try in the 78th minute but when the ball came off Lane from an offside position from a short kickoff from the Dragons, the Dragons were awarded the penalty and Widdop levelled the match at 10-10 at full-time, sending the match into extra time. Widdop became the villain after his kick out on the full, in an attempt of to nail a 40/20, which later saw Bulldogs' Josh Reynolds kick the match-winning field goal four tackles later in the 84th minute ending the Dragons season 11-10 at ANZ Stadium. Widdop finished the 2015 NRL season as the club's highest tryscorer and point scorer with him scoring 182 points in 24 matches, from 9 tries and 73 goals. At the conclusion of the club season, Gareth was selected for England for their test match against France and test series against New Zealand. In the test match against France, Widdop kicked 12 goals in a record 84-4 rout win of the 'Les Tricolores'. Widdop played in all 3 Test matches against New Zealand at five-eighth and kicked 10 goals in England's 2-1 Baskerville Shield series win.
On 13 February 2016, Widdop played for the World All Stars against the Indigenous All Stars in the 2016 All Stars match, playing at five-eighth in the 12-8 win at Suncorp Stadium.
In October, Widdop was selected in the England squad for the 2016 Four Nations. Before the tournament, England played a test match against France which saw Widdop tally his 100th test point in England's 40-6 win.
Minor Premiers: 1
World Club Challenge: 1