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.
The three man defence woukd easily favour them against Everton because Everton are not performing very well currently and would not be a problem for Arsenal to easily beat them but their problem cluld start when they face tougher opponents like PSV and Tottenham that have good wingers that would trouble their three man defence.
Everton at home has been a problem for Arsenal in previous seasons because they contributed to last season’s failure to win the Premier League, even though we know that they lacked squad depth last season. Nevertheless, with Everton’s current performance, I don’t think they can stop Arsenal from going home with three points, even if Arsenal play a three-man defense.
those two matches would be a big test for Arteta now he's currently facing an injury crisis from his defence line and I'll be suprised to see him win against PSV and Tottenham, i have a big doubt though and my prediction is that he'll win Everton, maybe draw PSV and lose against Tottenham because they're currently the toughest amongst those three they'll face next, well Arsenal is a big club and they could suprisenly win all three.
Arsenal is not facing any defensive injuries; Tomiyasu and Zinchanko are already back in form. The only player injured in Arsenal's defensive line is Julian Timber, and Arsenal succeeded last season without him. Therefore, even without Gebriel, Arsenal can win these two matches because they have defenders that can still be relied on.