After Avaí achieved promotion to the Campeonato Brasileiro Série A the same year, he departed for French side Lille.

At age 13, Gabriel began his career at Avaí but returned to São Paulo after a week due to homesickness.

However, he reconsidered his decision and returned to Avaí after two weeks, eventually going on to earn his first professional contract at age 16.

[12] On 31 January 2017, Gabriel Magalhães joined Ligue 1 side Lille, signing a four-and-a-half-year contract.

[16] The following season, Gabriel began to play more regularly, forming a centre-back partnership with José Fonte.

[18] Lille manager Christophe Galtier spoke about the defender's patience, saying "I saw he had great potential and he was able to seize the first opportunity he got.

[20] Interest in the Brazilian was reportedly high, with multiple clubs willing to match the valuation set by Lille.

[23] On 12 September, the opening day of the season, Gabriel made his debut for Arsenal, scoring the second goal in a 3–0 win against Fulham at Craven Cottage.

He had made a mistake that allowed Aleksandar Mitrović to open the scoring, but his goal in the 86th minute gave Arsenal all three points in a 2–1 win.

[40][41] On 17 September 2023, Gabriel made his 100th Premier League appearance in Arsenal's win against Everton at Goodison Park.

[45][46] He made his 150th Arsenal appearance on 11 February, scoring the third goal against West Ham in a 6–0 win at the London Stadium.

[47][48] On 15 September 2024, Gabriel headed in a 64th minute corner in the North London Derby, sealing a 1–0 away win for Arsenal.

[54][55] On 17 June 2021, he was named in the squad for the 2020 Summer Olympics, but was forced to withdraw with a knee injury on 6 July.

[56][57] In August 2023, Gabriel received his first call-up to the Brazil senior national team by interim head coach Fernando Diniz, for two 2026 FIFA World Cup qualification matches against Bolivia and Peru.

[62] According to his Lille teammate and mentor José Fonte, Gabriel "has to keep defending well by being aggressive, being well positioned, and not taking risks.