Jason Isbell and the 400 Unit (Derry deBorja, Chad Gamble, Jimbo Hart, Amanda Shires and Sadler Vaden) stopped by Jimmy Kimmel Live last night (Sept. 19) to perform “Cumberland Gap,” a new tune featured on Jason’s 2017 album, The Nashville Sound. Penned by Jason, “Cumberland Gap” finds the Alabama native returning to his rockin’ roots.…