A gatefold is an oversized page in a book folded to the same size as the other pages but intended to be opened out for reading or viewing. Magazine advertisements use gatefolds. So does the National Geographic.
A double gatefold has three parallel folds. The left and right edges of the paper fold inward and meet in the middle, without overlapping, along a centerfold.
You’ve heard of centerfolds. Now you know about gatefolds.