In "The Dragon and the Wolf" (Game of Thrones, S07E07),
the Night King and the army of the dead pass through a gap in the wall, which was made possible by the undead Viserion's fire.
If this much is possible, then why didn't they try it a long time ago on this side of the wall?
It seems that the Wall spans the land all the way to the coast on the east, but leaves a considerable gap on the west; a landscape known as the Gorge, where the closest castle on the Wall is Westwatch-by-the-Bridge.
Wildlings are known to use this circumvent the wall, and there is even a bridge.
Why didn't the army just use this gap to circumvent the wall, considering the events in S07E07 described above?