Board Thread:Theories and Speculation/@comment-25449165-20161013231440/@comment-3965545-20161014232934

Danzo said that it takes a Jonin to become Kage and that the Jonin needed to vote on who they deem worthy, he then said that there were no Jonin suitable to which Hiruzen threw Minato into the hat. So if Suna works like Konohagakure then Gaara most definitely became a Chunin, then Jonin. Also the Databook Mini says that Gaara was already 15 when he became Kazekage.