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.

Keith Dietrich
Stellenbosch, South Africa, 2007