Always a good place to start:

Most of the campgrounds are closed during the winter, except for a couple around Apgar/Lake McDonald Valley, as is the famous Going–to–the–Sun Road, and Many Glacier Hotel on the east side. Likewise, North Fork destinations will all be inaccessible by vehicle. But there's always skis! You can get good trip and backcountry information at the "local" Missoula REI, for whom GNP and its environs are their backyard.