Mystery of the Gate
From Halopedia, the Halo wiki
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The Mystery of the Gate glitch requires the Sputnik skull enabled before starting (a Sniper Rifle would help too). Go to Delta Halo on any difficulty and progress until you get to the part where you must go down the sloping ramp into another level. Get close to the door, and Grenade Jump to one of the ledges on your left or right, then grenade jump again to the top of the building. Make your way across to the opposite side, where you will have to grenade jump to the platform below (this is the spot where you would fight the Hunters). Go to where the Gondola would dock and zoom in and look slightly to the right; there will be a floating switch!
Any enemies that you encounter on the way, except the first two Jackal Snipers, will not be able to see you.
The Truth
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The gate is most likely the switch to activate the gondola glitched so that it appears early. To explain this it would mean bungie had placed it there to only appear when the gondola is there and dissapear after it is activated, through the skulls and glitching the game it appears.
