GNR Plaza is the focus point of a battle between super mutants and the Brotherhood of Steel. The area consists of a sculptured fountain in the center and a caved-in metro entrance. The entrance to the station is fortified with sandbags. Several Brotherhood members stand guard at the steps and on balconies off to the side. During this quest, one of the five known super mutant behemoths will spawn behind a wrecked bus that blocks the alley from which the behemoth spawns in. It will then proceed to destroy the bus, taking out Reddin as well, before attacking you and the surviving BoS.
The main lobby of GNR is heavily fortified with sandbags and other barricades as well as several named Brotherhood of Steel members armed with flamers, missile launchers and laser rifles. To the left and right are doors leading to the balconies outside as well as some bathrooms. There are some first aid boxes and ammunition boxes scattered throughout the lobby.
Upstairs, there are some offices and the recording studio in which Three Dog resides. Another door leads to the back exit, which is inaccessible from the exterior.
If Sarah Lyons is currently a companion, a dead Brotherhood of Steel Knight carrying a Fat Man and eight mini nukes can be found outside GNR during or after the encounter with the super mutant behemoth. He will still be there even without Sarah Lyons, but he will be carrying standard Brotherhood equipment.
It is possible to find a picture of the building under the notes section of the Pip-Boy 3000.
Prior to fending off the super mutant horde, the main doors will be locked and inaccessible. Just 'buzz' the intercom after the super mutant behemoth is defeated and the door will be unlocked.
It is possible to skip the need to come to GNR Plaza during the main quest by talking to Dr. Li about where the Lone Wanderer's father went. If she's spoken with before Three Dog, she will inform the player character about Jefferson Memorial, triggering the next phase of the main quest.
If one talks to Pinkerton before coming to the Plaza, then Sentinel Lyons and all the other BOS that would normally be there won't be. However, the terminals at the Citadel still list initiate Reddin as KIA, even though the battle never occurred.
Walking into Chevy Chase can trigger the battle of the Galaxy News Radio's plaza.
The other way in is the eastern side entrance of the building which requires some climbing, as there is no easy way to this side. Once on the side, there is a locked door (very hard) that will result in lost Karma if picked.
The area where the behemoth comes from winds around the GNR building and into a dead end, so it is unknown how the behemoth gets into the GNR plaza.
In the GNR studio there is a room with a hole in the floor and the ceiling and a lot of debris laying about. Through the hole in the ceiling you can see the radio tower and another door. It's possible to stack the debris and jump up through the hole in the ceiling where you find the door is 100 skill locked. Behind the door is a third floor that is unfinished by the game authors so don't waste time trying to get to the door.