The Galaxy News Radio building and GNR Plaza are located in the Chevy Chase district. The first and easiest way is to make your way to Farragut West station and go through the utility tunnels. After a brief fight with some ghouls, the player should find themselves in the Tenleytown/Friendship station tunnels. Following the tracks to the Tenleytown half (go right), the player will find two super mutants fighting Lyons' Pride in an alley.
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.
It is possible to find a picture of the building under the notes section of your Pip-Boy 3000.
Prior to fending off the super mutant horde, the main doors will be locked and inaccessible by the player. Just 'buzz' the intercom after the super mutant behemoth is defeated; the door will be unlocked for you.
It is possible to skip the need to come to GNR Plaza during the main quest if you talk to Dr. Li about where your father went. If you go to Rivet City and speak with her before Three Dog, she will inform you about Jefferson Memorial, triggering the next phase of the main quest.
If you skipped coming to the Plaza by talking to Pinkerton, then Sentinel Lyons and all the other BOS that would normally be there won't be, and if you check the terminals at the citadel it still lists 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.