More precisely, the one pictured here is an “open” gatefold.
In a gatefold 2 panels fold in toward the center to meet in the middle, creating the appearance of a gate.
That seems pretty clear, doesn’t it?
Still, many also call this a “double gate.” Yet one panel folding in is not a single gate, so two panels folding in is not a double gate. (Sorry to disappoint, it’s just a gate.)
In an “open gatefold” the “gates” meet in the middle and go outward again.
And of course there is no rule that says the gate has to close in the middle. Move the opening to the right or to the left to create more interest or to help guide the reader through the printed piece. (PRO members, discover loads more about how you can harness the power of the gatefold in this exclusive PRO Tip!)