Neither can you find, nor can you edit the materials without any additional programs.
These are your saviors in that matter:
A program used to open GCFs "Game Cache Files", which are actually folders containing all the games files.
Get it here:http://nemesis.thewavelength.net/index.php?p=26
While browsing the gfc, this program is used to open VTFs "Valve Texture File" These are the materials and sprites used in your game of choice.
Get it here:http://nemesis.thewavelength.net/index.php?c=178