Gubeikou is a Great Wall section that most Beijing visitors walk straight past on their way to Simatai. That is their loss. The wall here is genuinely ancient in places — much of it predates the Ming rebuilding that produced the sections everyone photographs — and the restoration is minimal. You walk on actual centuries-old stone, through actual centuries-old towers, with almost nobody else around.
The setting in the Baihe River valley is also beautiful, with the wall climbing steeply from the valley floor to the ridge and the river winding through the gorge below. A small village at the entrance has local guesthouses where you can stay for next to nothing — a fraction of the Gubei Water Town hotel prices. The lack of cable cars, interpretation centres, or organised tourist facilities is, depending on your disposition, either the point or a problem.
Gubeikou makes a logical complement to Simatai: the two sections are close enough to combine in a single day if you have transport, and the contrast between the ultra-managed Simatai experience and the very raw Gubeikou one tells you a great deal about the range of ways China is handling its most famous monument.