From the Glacier Gorge trailhead parking area, hike generally SW until a footbridge takes you across Chaos Creek and continues SW to a trail intersection. The trail coming in from the right comes down from the parking for Bear Lake. Turn left and head for Alberta Falls in .5 mile. Above the falls, the trail winds and switchbacks by the "Glacier Knobs" to another intersection at 1.4miles from Alberta Falls or 2.2 miles from the TH. Take the left fork toward Glacier Falls and Mills Lake which is about 2.8 miles from the start. Jewel Lake is an additional .4 mile beyond Mills. The backcountry campsite (Glacier Gorge #39) we were able to use is at about 3.8 miles in and is on the west side of the valley. The turnoff is marked. Continuing past the turnoff for the campsite, you'll reach Black Lake at 5.4 miles in from the TH, so the campsite is 1.6 miles from the lake. For the last mile or so in, expect the trail to not be as well used, with some boggy sections, elevated planks, some cairns and some rebuilt sections. The maintained trail terminates at Black Lake.
Designated backcountry campsites only. This link is to the only designated campsite in Glacier Gorge. You must obtain a permit to use. There is only one designated site here that can accommodate two tents. There use to be an outdoor privy, but the most recent information on the park websites instructs campers to use Wag Bags like a Restop 2. The turnoff for this campsite comes a short distance after the trail crosses a large, exposed dome of rock with a single, large glacial erratic boulder sitting on the rock dome.
http://www.nps.gov/romo/planyourvisit/upload/039-Glacier-Gorge-2012.pdf