Louisville has the megacavern but very little gravel nearby. There is a 40 mile gravel route called Leavenworth about a 45-minute drive west of Louisville in Indiana but I haven't ridden it. Also, the mountain bike trail network in Brown County, IN, about 60 minutes north of Louisville, is pretty widely recognized as the best mountain bike network in the Midwest.
If you’re talking about the Leavenworth route on gravelmap.com I put it there. It’s pretty nice, about 60% gravel I’d say and nothing too rough. Several small climbs, some of which are on the paved sections. All the pavement sections except for the last two miles back to the start have basically zero cars.
I did another one today out of Madison, IN that was about 55 miles, roughly 50% gravel with a couple of very steep climbs. One of them was on pavement and out of the saddle the whole time, the other fairly steep as well but on gravel. It’s not on gravelmap yet but if you want the file let me know.
As jmeckelnborg said, there is no gravel really close to Louisville but there is excellent road biking just across the river in Indiana with very little car traffic and two options for crossing the Ohio on either the Big Four Bridge or the East End Bridge.