You know in your steam folder, the normal one, there is also a TF folder?
The .gcf is like a resvoir, when any files aren't in that tf folder the game goes to the .gcf for the files, therefore you can't edit it or else bad shit would happen when people like you try it and succeed.
In your steam folder, which should be in program files somewhere, then in steamapps, your steam username, not your nickname, the one you log in with and can't change, should be team fortress 2, and in that, tf. In that tf folder goes your materials, models, sounds, and other folders. The .gcf can be used when you want to get the original files.