Both the Gunners and Valencia confirmed the transfer via statements on their official websites, with the defender signing a five-year contract with an 80-million euro release clause.
The move brings to an end Gabriel's short tenure in north London, with the 26-year-old having failed to settle at Arsenal after moving from Villarreal in January 2015.
The Brazilian defender has not featured for Arsene Wenger's side since sustaining a knee injury in Arsenal's 3-1 win over Everton on the final day of the 2016-17 campaign.
The move follows Arsene Wenger repeatedly admitting that he needs to offload a number of squad players in order to make room for the likes of Alexandre Lacazette, Sead Kolasinc, as well as youth promotions such as Reiss Nelson.
Gabriel is set to be presented to Spanish media tomorrow.
