The 22-year-old moves to the Emirates Stadium as Mikel Arteta’s fourth summer signing.
Arsenal have already snapped Willian from Chelsea on a free transfer while loan moves for Cedric Soares and Pablo Mari have been made permanent from Southampton and Flamengo respectively.
Gabriel has been courted by some of Europe’s biggest clubs this summer after putting in some superb performances for Lille in the 2019/2020 season.
The former Avai defender made 34 appearances in all competitions as Christophe Galtier’s men finished fourth last term.
On the news, Arsenal’s technical director Edu said: “We are delighted to have signed Gabriel.
“He is a player we have been studying for a while and he was in demand from many clubs, so we are proud to have agreed and completed this transfer with Lille and the player.
“Gabriel is a player of high quality and Mikel and his coaches are looking forward to integrating him into the club.”
Gunners head coach Arteta added: “We welcome Gabriel to Arsenal. He has many qualities which will make us stronger as a defensive unit and as a team.
“He has proved with Lille that he is a defender with many outstanding attributes and we are looking forward to watching him grow as an Arsenal player.”
Gabriel increases Arteta’s central defensive options to eight, with the likes of Sokratis, Calum Chambers, David Luiz, Shkodran Mustafi, William Saliba, Mari and Rob Holding still at the club.
Gabriel will wear the number six jersey.
