I'm in Cobb County and am familiar with a small subset of gravel roads up north. The one's I'm familiar with are going to be steep and/or long climbs.
If you don't mind climbing then there is the Southern Cross route. You could do loop from the Springer Mountain AT trailhead parking lot for 19.6 miles and 2084 feet climbing. The long climb up Forest Service Rd 54 is along a rushing, whitewater creek/river.
https://ridewithgps.com/routes/27706243
There are lots of nice routes south of Atlanta near Monticello. Part of the Red Clay Ramble route would be nice.
https://ridewithgps.com/trips/6192556