[5] The same year, he was promoted to the first-team in order to replace Jores Okore who had signed with Aston Villa.

[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.

[15] 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.

[19] On January 22, it was confirmed that Gregor moved to Swedish Halmstads BK on a deal until the end of 2027.