During his career, Robertson played for Nottingham Forest, Rotherham United, Chesterfield, Crewe Alexandra and Northampton Town and Grimsby Town. He has also represented Scotland at U21 level.
Born in Edinburgh, Robertson started his career as a junior in Heart of Midlothian's youth system but moved south of the border to join Nottingham Forest when he was 17 in February 2001.
After progressing through the Red's academy, Robertson made his debut in September 2003 against Tranmere Rovers in the Carling Cup, which Forest won on penalties after a goalless draw.
He made his Championship league debut against local rivals Derby County four days later as a late substitute in a 1-1 draw at the City Ground. His full league debut came the following week against Preston North End in a 1-0 home defeat. On 28 May 2004, boss Joe Kinnear offered Robertson a new one-year deal. On 10 May 2005, he was released by manager Gary Megson.
Robertson went on to make a total of 44 appearances for Nottingham Forest.
In June 2005 Robertson signed for League One Rotherham United on a free transfer. He made his debut in a 1–2 home defeat to Walsall in the opening fixture that season.
Robertson spent 2 years at the club making a total of 56 appearances and scoring 1 goal in a 2-0 victory away to Swansea City at the Liberty Stadium.
"Gregor can operate at left-back or on the left of midfield and will add much-needed steel to the Millers squad, he is strong, athletic and is the kind of young, mobile player I am looking for."
Robertson joined League Two side Chesterfield on a two-year contract in June 2007. He made his debut in the Spireites opening league fixture of the season, a 0-0 draw away to Chester City. He netted his first goal for the club in a 3-1 win away at local rivals Mansfield Town that season, scoring direct from a corner kick.
On 2 July 2009, he signed a new one-year deal, which kept him at the club until July 2011. On 4 October 2009 he scored an equaliser in the 15th minute in the 2–2 stalemate against Morecambe, latching on to Darren Currie's cross and volleying home from close range.
In November 2009, Robertson suffered a double leg break in an innocuous tackle in a Johnstone's Paint Trophy tie at home to Carlisle. After a year on the sidelines he returned to action in November 2010 in a 2–0 FA Cup 1st round victory away to Harrow.
On 8 June 2010, he signed a new one-year deal. Robertson went on to make a total of 23 appearances that season in Chesterfield's 2010/11 League Two winning campaign, enjoying a successful spell at centre half for much of it. However, he suffered an Achilles tendon rupture away to Hereford United in April, ruling him out for the last few games of the season and much of the next.
Robertson returned to action in League One the next season in a 1-0 Boxing Day defeat away to Huddersfield Town and was part of the Johnstone's Paint Trophy winning squad that season.
Robertson made a total of 133 appearances for Chesterfield, scoring 3 goals.
In July 2012, Robertson joined League One side Crewe Alexandra, signing a two-year deal.
On 16 January 2014, Gregor had his contract mutually terminated by the club after making 40 appearances in all competitions for The Alex.
On 4 February 2014, Robertson agreed to join League Two side Northampton Town on a deal until the end of the season. He played 15 League games in the club's successful battle against relegation, with the Cobblers guaranteeing their League Two status with a 3-1 victory over Oxford United on the final day of the season. Robertson signed a new one-year contract on 23 May 2014. Gregor was released by Northampton Town on the 20 January 2015.
On 26 January 2015, Robertson signed for Conference Premier side Grimsby until the end of the season.
On 11 April 2015, he injured himself in the first half and had to be substituted at half time, in the 0–1 defeat against Wrexham. Scans revealed he had damage to his medial ligament meaning his campaign was likely over. Surprisingly, he made a comeback to the starting lineup, due to an hamstring injury sustained to Josh Gowling, Toto Nsiala reverted to his favored centre-back position having previously played at right-back while Robertson was injured, but during the 2015 Conference Premier play-off Final against Bristol Rovers at Wembley on 17 May 2015, he had to be substituted in the 72nd minute, being replaced by Danny Parslow; following a 1–1 draw after extra time, Grimsby lost 5–3 in a penalty shoot-out.
On 29 May 2015, Robertson agreed a new one-year deal with the club. He played in Grimsby's 3–1 victory over Forest Green Rovers in the 2016 National League play-off Final at Wembley, seeing Grimsby promoted to League Two after a six-year absence from the Football League. Roberston was released when his contract expired at the end of the season.
Following his release from Grimsby, Robertson announced his retirement on his Twitter account.
Robertson won a total of 15 caps, scoring two goals for Scotland U21s, including one match against Spain's U21 side featuring FIFA World Cup: 2010 winners Andres Iniesta, Sergio Ramos and Cesc Fabregas, which the Scots lost 3–1.
On 16 August 2005, a Shaun Maloney corner was flicked on by club-mate Craig Beattie for Robertson to head the opener in the 1–3 comfortable win against Austria U21s. On 7 October 2005, he scored a header to put Scotland U21s into a 2–0 lead on 44 minutes, eventually losing 2–3 against Belarus U21s.As of match played on 15 May 2016.
Robertson studied a three-year degree in Professional Sports Writing and Broadcasting, Journalism at the Staffordshire University, and worked at the Nottingham Post as part of a work placement, writing articles for them.Chesterfield
League Two Champions, Promoted (1): 2010–11
Football League Trophy Champions (1): 2011-2012
Football League Trophy Champions (1): 2012-2013
National League Play-offs Winners (1): 2015–16