The Brazilian defender has penned a five-year contract at the La Liga side and leaves for £11m.
Gabriel will join Neto and Nemanja Maksimovic as the new faces hoping to change Valencia's fortunes.
During his time at the club the Brazilian failed to fully establish himself as a regular at the back and was sometimes the focus of criticism aimed at Arsenal's defence.
Arsenal defender Gabriel looks certain to leave the club after he landed in Valencia to complete his switch to the Spanish club.
The move brings an end to two-and-a-half seasons at the Emirates Stadium, with the centre-back moving from Villarreal in January 2015.
"We would like to thank Gabriel for his contribution to the club and to wish him well for his return to LaLiga with Valencia", an official Arsenal statement read.
While in north London he picked up two FA Cup winners medals and two Community Shields.
Gabriel sustained a serious knee injury in Arsenal's final Premier League game of last season and did not feature in pre-season.
Arsene Wenger is keen to streamline his 33-man squad with Carl Jenkinson, Mathieu Debuchy, Kieran Gibbs and Jack Wilshere also expected to go.