There is no key forged in steel or wood. Instead the key is in fact a type of destiny. To open the gate you must close a path of your life behind you; venturing down a road where you cannot turn away from. The physical gate will stay forever open and the decision you have made will keep a potential path for your life will slam closed and remain forever closed.
You must immerse the Bear in the ethereal beauty of the ancient universe, a beauty captured in a jar. It brings both light and creation but also darkness and destruction. The bear climbs the mountain but does not climb it to experience this beauty; it knows it is up there but avoids it for fears what would happen to its paws if it grasped it in its claws. You must give the Bear the gift but be aware it will not celebrate the gift and not honor its sender. The beast will roar and the gate will open.
*Long story short, you have to nuke the NCR in Lonesome Road. Then you can go past to a nuclear devastated NCR settlement for loot.*