Sunrift Gorge is on Baring Creek just above the falls; cap off the Sun Point [1] walk by taking the spur trail and climbing to the gorge. Or, follow the lead of most gorge viewers and park in the Sunrift Gorge pullout and walk 50 yards up the path to the narrow chasm, formed not by erosion but by a vertical slip of the rock.
Links:
[1] http://www.moon.com/destinations/montana/glacier-national-park/logan-pass-st-mary/hiking/hikes-around-st-mary-lake/sun-point