Symbolism, Imagery, Allegory
Trust us: you probably wouldn't enjoy a vacation to Gatlinburg, Tennessee. Unless you love tacky tourist towns, that is, in which case you'll love it.
Bryson has actually been to Gatlinburg before (and wrote about it), but the town has changed a whole lot since then. He actually lays it out pretty straight for us:
"In 1951 [...] Gatlinburg had just one retail business [...] By 1987, Gatlinburg had sixty motels and 200 gift shops." (1.8.12)
And it's not even like these are cool mom-and-pop operations—they're either major corporate chains or money-hungry tourist traps.
In this way, Gatlinburg represents the negative aspects of modern American culture. Specifically, it represents the way that commercialization has changed the country, as this tiny town has been transformed into a money-grubbing monstrosity in mere decades. Benton MacKaye (the founder of the AT) would roll over in his grave if he saw how this influx of money has changed the face of this once-quaint entryway to the Appalachian Trail.