Thegoalmac reported earlier this week that the Gunners were confident of pushing through a deal for the Brazilian, despite the likelihood that he will fail to get a British work permit at the first time of asking.
And the Spanish club have now Tweeted that a move is in place : "Deal agreed with Arsenal for Gabriel, who will say goodbye today before the match [against Levante]. Good luck!"
The Yellow Submarine also confirmed that they will take Costa Rica international Campbell on loan at El Madrigal until the end of the season: "Welcome Joel Campbell! Loan deal agreed with Arsenal until the end of the season."
The 22-year-old had hoped to break into the Arsenal line-up this season, but has seen just 41 minutes of Premier League action, with Alexis Sanchez, Danny Welbeck and Olivier Giroud providing fierce competition.
Gabriel will become the Londoners' second January signing, after 17-year-old Pole Krystian Bielik was brought in on Friday.
