As relayed by Sky Sports News HQ, the centre-back suffered medial ligament damage during the final game of the Premier League season against Everton, which Arsenal won 3-1 on Sunday:
It's a huge blow for manager Arsene Wenger, as he is already without centre-back Laurent Koscielny for the Wembley Stadium showpiece due to suspension.
Gabriel hasn't always convinced defensively during his time at the Emirates Stadium due to his questionable positioning and occasional ill-discipline.
Further, Arsenal's recruitment of Germany defender Shkodran Mustafi last summer ensured he is no longer a candidate for a first-choice slot in the back line alongside Koscielny.
However, he is also among the Gunners' quickest centre-backs, good in the air and a composed passer of the ball.
Injuries often prove costly to Arsenal given how many they seem to gather each season, and as they aim to salvage their season with glory in the FA Cup, this is a significant setback.
It means Wenger has a big decision to make at the back on Saturday. Youngster Rob Holding is likely to start having impressed as of late, while Per Mertesacker was introduced as a late substitute against Everton for his first appearance of the season.