The Gunners have agreed a £10m deal for the Brazilian defender, who has been on Manchester United's radar, while fringe striker Joel Campbell will join the Yellow Submarine on loan until the end of the season.
Wenger believes the deal will be completed tomorrow after being held up by work permit issues.
After the 3-2 FA Cup win over Brighton, he told BT Sport: "It's very, very close and it should be done tomorrow.
"He has good size, good pace, a good defender who has played only one year in Europe. There will be some things to work on but potential is there."
Villarreal head coach Marcelino yesterday admitted he was surprised by the move.
"Gabriel’s departure surprised us all because we didn’t expect to make changes to the squad [in January]," he revealed.
"From a sporting point of view it’s unfortunate, but neither the club nor the player could afford to resist Arsenal’s offer. I wish him the greatest professional success."
