Go to the map named "of3a1". This is the map where you see Freeman running into the teleporter. Follow him into that same teleporter. After the map finishes loading, you will see yourself falling. Normally, if you let yourself fall the game will restart at map "of3a1". However, if you enable the /noclip and go to the nearest platform in front of you, Gordon Freeman will appear. Gordon just stands there with no weapons, and wears silver sunglasses. It takes some time to kill him with just bullets -- try grenades or a strong weapon. Alternately, you can go to this location enabling the map of3a1b code.