Gabriel sustained what appeared to be a nasty injury in Arsenal's pre-season friendly against Manchester City on Sunday.
The Brazilian was on the ground for several minutes of the 3-2 win for Arsenal with what looked like an ankle injury following a challenge with City striker Kelechi Iheanacho.
"He is in the medical team with our doctor and our physios," manager Arsene Wenger told Sky Sports News HQ after the match.
"He's in pain and let's hope the pain is not a sign of bad news but certainly it's an ankle sprain, we just need to wait to see how bad the damage is.
"Of course we are frustrated but I've never seen an intelligent accident. We have to take that. It's part of our game."
Gabriel is Arsenal's only fit senior centre-back with Laurent Koscielny and Per Mertesacker out injured and started Sunday's friendly alongside Rob Holding, a £2m purchase from Bolton Wanderers, at the heart of the Gunners' defence.
Arsenal have made bids for Lyon striker Alexander Lacazette and Valencia centre-back Shkodran Mustafi as they bid to bolster a squad which finished second in last season's Premier League.
The injury to Gabriel blighted another otherwise good friendly for Arsenal, who won 3-2 with goals from Alex Iwobi, Theo Walcott and Chuba Akpom cancelling out efforts from Sergio Aguero and Iheanacho.