The Gravity Throne Hologram Glitch can be seen in the Halo 2 campaign level Delta Halo. To do this, you need a Ghost and much patience. One way is to do The Delta Halo Vacation, and you should be able to reach it at the end of the level. However, no Honor Guard Elites will spawn and attack you, and the hologram Prophet that Cortana translates is gone. But mysteriously enough, the Gravity Throne is still there. The reason is unknown, possibly because he counts as an enemy like all other enemies, and so didn't load into the game. The other hologram near the ruins is the same as this too.
Another method: Perform the Floating Switch method. When you are on top, go near the center. You will be able to see the weird structure below. Jump down and if you land correctly, there will not be any Honor Guards, and the Gravity Throne will be there without Regret.