Well it's actually in 3 states, KY, TN and VA. Boone found it while traveling on what they used to call the " Wilderness Road". The Gap is thought to have been created by a meteor impact. By the way, thank you History Channel, 'cause I it ain't like learned any of this in publik skool!
I just read about that in the book SALT. He followed trails laid out by buffalo and deer going to salt licks. I think it was West Virginia over the mountains to get to the Ohio River etc. Just remember the natives knew all these routes.