The Brazilian was absent as Arsenal beat Chelsea 3-1 on Saturday because he began a self-isolation period after coming into close contact with someone who had tested positive for Covid-19.
Arsenal confirmed on Monday that Gabriel has now also tested positive, meaning he will miss at least two more matches.
Mikel Arteta’s side, who are currently 14th in the Premier League, face Brighton on Tuesday and West Brom on Saturday.
‘Gabriel has tested positive for COVID-19 and is isolating according to UK Government and Premier League protocols,’ Arsenal said in a statement.
‘We will continue to monitor and support Gabriel, and we are hoping he will be available again after our next two matches.’
Gabriel, 23, has largely impressed for Arsenal following his move from French club Lille last summer, making 15 appearances for the club in all competitions.