"The gorge" refers to the area east and south of Devil's Den, culminating in the area know as the Slaughter Pen.
Most people go to the Den and think that the battle took place on the big rocks themselves. In truth, the rocks (and specifically Smith's Battery) were the objective, but most of the fight for the Den took place in the gorge, the Triangular Field, and in the Valley of Death / Slaughter Pen area.
Because of the demolition needed to place the road around the Den, the continuity of the scene has been broken up. If you go there, try to picture the area without the road, and you see that the low area below the big rocks (the gorge) was a poor place to march over in the face of fire. That's why I want to take that approach.