The Gunners pair climbed to meet Declan Rice's inswinging free kick midway through the first half against Athletic Bilbao, but as Gyokeres made contact with the ball, Gabriel made contact with the back of the striker's head.
As Mikel Arteta stood on the touchline grimacing at a nasty-looking collision, Arsenal medical staff rushed onto the pitch to treat Gyokeres, with blood soon seen pouring from the back of his head.
Gyokere was thankfully OK to continue, and Gabriel quickly apologised for his involvement.
Arsenal began their Champions League campaign on Tuesday evening at the Sam Mames, taking on Athletic in their first League Phase game.
Arteta & Co. will be looking to go one better than last season’s semi-final exit, at the hands of eventual winners Paris Saint-Germain.
Eberechi Eze was given his Champions League debut, a few days after providing an assist on his first Premier League start for the Gunners.