First, open up the dev console by pressing the ~` key (below esc on most keyboards). When the console pops up, type ‘map c1a1c’ into it. After the map loads, you will have to enable cheats. To enable cheats, type ‘sv_cheats 1’ into the console. After that type in ‘give item_suit’ to get the H.E.V. suit (when you warp to a map via dev console, you will not spawn with the suit or any weapons, and you will need the suit’s flashlight to see in the Gaben room, since it is completely dark) (if you do not get the suit, re-type ‘map c1a1c’ into the console and try again). You will also have to fly through walls, so type 'noclip' into the console.

Now, fly to the bottom of the large diagonal elevator, turn left, and fly through the wall. You should see a black cube in the empty space. Fly into it, turn on your flashlight, and congratulations! You are now in the Gaben room!