The Glacier Divide Route parallels the Continental Divide through Glacier National Park for about 125 miles, usually off-trail. The heart of the route is between Brown Pass in the north and Firebrand Pass (or possibly Little Dog Mountain) in the south. The most logical access points for a thru-hike are Waterton Lakes and either Marias Pass or East Glacier, Montana.
The roots of the Glacier Divide Route start with J. Gordon Edwards, who described a Continental Divide Route — often in such vague terms as to be non-actionable — in his definitive Climber’s Guide to Glacier National Park. The entire route has been completed before, at least in segments; I can’t find record of a thru-hike, but it’s likely been done.
For me this is an unfinished project. Dave Chenault and I attempted the route in July 2016, but for various reasons only finished about three-fourths of the route before exiting. It’s a legit route and I expect to return next summer to finish it up.