In double gatefolds there are three parallel folds. The left and right edges of the paper fold and meet in the middle, without overlapping, along a center fold.
The outer panels (the ones that fold in to the middle) are usually 1/32" to 1/8" smaller than the inner panels (the ones covered by the panels that fold in) to allow for proper folding and nesting.
Some menus might use a double gatefold or a modified version where the outer panels are one-half to one-quarter the size of the inner panels. Advertising pieces that double as posters, some brochures, and inserts in books or magazines use this style of fold.