A gate can be either open or closed. If open, one can pass through. If closed, it becomes a barrier. Nevertheless, there is always a way to overcome such a barrier, by climbing over it or tunneling under it or breaking the lock, etc. But when the gate is a gateless gate, no barrier presents itself, in which case there is nothing to climb over, tunnel under or break. Consequently, the gateless gate is impenetrable.
The Japanese term for such a gate is mumon. "Mu" is a negative term, meaning no, not or without. "Mon" means, simply, gate. The masters of Zen Buddhism composed little stories or prose poems, called koans, that function as gateless gates. If the student can penetrate the koan, the gate will open. But the koans are impenetrable, so the gate will always remain closed.
Franz Kafka wrote of a man who stood at such a gate for most of his life and was never able to pass through. The Spanish filmmaker Luis Bunuel created a film about the gateless gate, The Exterminating Angel. It's about a group of people who gather together for a dinner party but, when it's time to leave are unable to do so. Nothing prevents them from passing through the door, which presents no barrier, as it isn't locked. But for them it becomes a gateless gate and they remain trapped inside for months. In the final scene we see a herd of sheep, which gives us a hint as to the nature of this gateless gate.
When I saw President Obama's speech before congress last night I was reminded of this film. Everyone seemed to be having such a great time (except for the Republicans of course) and our president was literally beaming with confidence. However, as is becoming increasingly evident, the barrier confronting them is not one that can be climbed over, tunneled under or smashed through. Like the door of the dinner party or the Zen koan, it is a gateless gate. The wealthy, privileged and spoiled partyers of Bunuel's film start the evening happy and carefree, but as the film progresses, food and water become increasingly scarce and the social order disintegrates.
The koan has been described as a hot coal stuck in the throat that one can neither spit out nor swallow. The Gateless Gate is the Gate of Hell.