This is a pleasent weekend route and provides a great tour de GNM. It can be done from either end but requires two vehicles or some creative backtracking. I'll describe the route from north to south.
Total mileage is about 18 miles plus or minus side trips to interesting panoramae. It is doable as a one-day sprint hike, but splitting it into two or three days makes a nice, social, low-key backpacking trip.
Starting from US55 (see Getting There on the main page), hike south on the Tuscarora trail (blue blazes) on easy, gentle terrain passing occasional viewpoints and a powerline crossing. You're straddling the WV/VA state line and can choose from one moment to the next which state you'd rather be associated with.
From US55, it's 4.5 miles to the Gerhard shelter. This makes a good campsite if the timing works out that way.
From the shelter, continue south on the increasingly narrow ridgeline. Enjoy the views of Paddy Mountain and Vance's Cove to the east and the Capon River Valley to the west. After a mile of ridge, you start to descend for three miles to Waites Run Road via a series of broad switchbacks and an old fire road. Cross the Waites Run (good water location) and start back up the other side of the valley on the Pond Run Trail (blue blazes). Technically, you're now on Mills Mountain, but it's all part of the greater GNM massif.
Three miles of pretty ascent through a stream valley brings you to a T-intersection with the Halfmoon trail (yellow blazes, right) and the continuation of the Tuscarora/Big Blue trail (left). There is a decent campsite straight ahead with a good view to the west. The Halfmoon Lookout to the west is supposedly a very nice destination if time and inclination permit.
If the day is still young, bear left and continue up Mill Mountain over some ankle-turning rocks. Slightly less than a mile after the intersection with the Halfmoon trail, you'll top out the mountain and come to a right intersection with the Mill Mountain Trail (orange blazes). The Tuscarora trail continues ahead to the Sugar Knob Cabin (which is locked unless you've made prior arrangements). There is a fantastic campsite here at the intersection amongst some large trees and large rocks.
On the Mill Mountain trail, continue south along a gradually descending ridge with occasional views. A good spring is passed about 1.4 miles from the summit flowing across the trail. More ridge walking gradually loses altitude and, in an additional 1.5 miles, you encounter a side trail (left) for the Big Schloss Cuttoff trail (blue blazed). This steep descent drops you down to a fire road and eventually civilization.
Continue on the ridge another mile to Big Schloss (which is hard to miss) and thence 2 miles precipitously down to the Wolf Gap Rec. Area where (hopefully) you have another car waiting.
Water is the critical factor on this hike. There is reliable water available at the Waites Run and at the spring on Mill Mountain. The Gerhard shelter has a spring, but it is about a quarter mile down a steep hill and is nothing very impressive. In summer and fall, water may be infrequently available. Plan accordingly.
For a detailed account of an October, 2001, expedition, see this trip report
Nothing special. The trail is in good enough condition that light hiking boots are plenty. Bring lots of water and equipment for purifying more. PATC Map F is an excellent resource for planning this trip.