According to the latest news, I saw that he did not have a serious injury. That's good news for him, Arsenal would probably struggle without him.
That is a very good news for Arsenal and Brazil as the injury is not serious, because he is a very good defender with great talent.
But unlike last season, Arsenal cannot suffer without him because as you can see he did not start almost first three matches this season for Arsenal, Arsenal now have backup for all positions which is the problem they had last season that cost them the league.
Their next game is against Everton and they need to defend well. If one of the players is missing from the team, their chances of conceding a goal are much higher. Even though Everton started this year badly, the team needs to get a comfortable win and be highly motivated for the next games
William Saliba and Kiwior can manage the back even if Gebriel is injured, moreover, Arsenal can still play three defenders and have a perfect game since Everton did not have fast wingers that can give Arsenal problem.