Blake Shelton and Gwen Stefani: Hangin' In Nashville

Gwen Stefani took a private jet to Nashville Tuesday with a male passenger in his late 30's ... Blake Shelton ... TMZ has learned.

Blake will be performing Wednesday night at the CMAs. Gwen has no reason to be in Nashville, other than the fact that Blake is there.

We're told they have not decided whether Gwen will show up tonight, but if she does it will be more than interesting, because Miranda Lambert will also be performing.

Multiple sources close to Gwen tell TMZ, she and Blake are not "in a relationship right now," but all acknowledge they're hanging out a lot. One source said, "They could be in a relationship in the future but right now they're just good friends." The source did not define "good friends."