A centre-back, Gregor joined the FC Nordsjælland academy in 2011 from Vanløse IF.[4] In 2013, he captained their under-19 side, winning gold in the under-19 league. Gregor was also announced as Talent of the Year in FC Nordsjælland in 2013.[5] The same year, he was promoted to the first-team in order to replace Jores Okore who had signed with Aston Villa. In December 2013, his good performances were rewarded with a four-year contract extension.[6]
Upon his contract expiring, Gregor refused to sign a contract extension, making him a free agent in January 2018.[7]
Helsingør
On 4 January 2018, Gregor signed a two-year contract with FC Helsingør.[8] He made his debut for the club on 11 February as a starter in a 6–1 away loss to OB.[9] Gregor made 45 total appearances for Helsingør in which he scored four goals.[10]
Loan to Haugesund
On 2 August 2019, Gregor joined Norwegian club FK Haugesund on loan for the remainder of 2019.[11][12] He made his debut for the club on 4 August, coming on as a first-half substitute for the injured Fredrik Pallesen Knudsen in a 1–1 away draw against Stabæk.[13] Gregor scored his first goal for Haugesund on 20 October, scoring on a header in a 3–1 loss to Molde.[14] He ended his tenure with the Norwegian club with one goal in seven appearances, as he struggled to break into the team.[15]
Lyngby
On 19 December 2019, Lyngby Boldklub confirmed that Gregor would join the club January 2020 on a three-year contract.[16] Upon his return to Denmark, he made his first appearance for the club on 17 February 2020, as a starter in a 2–0 away loss to FC Midtjylland.[17] He suffered relegation to the Danish 1st Division with the club on 9 May 2021 after a loss to last placed AC Horsens.[18]
Gregor scored his first goal for the club on 18 March 2022, heading in a cross from Magnus Kaastrup to secure a final 3–0 score in the away match against Esbjerg fB in the 1st Division.[19]