Instructions
Download and install GCFScape by double-clicking on the installer and following the instructions.
Launch GCFScape by clicking on the application's shortcut in the "Nem's Tools" folder of your Microsoft Windows "Start" menu.
Click on the "Open" button and navigate to the following folder on your hard drive:
C:\Program Files\Steam\steamapps
Double-click on the "source materials.gcf" file to open it. This file contains all "Half-Life 2" textures.
Click on the "+" sign next to "hl2" in the left navigation pane to expand that folder. Then, click on the "+" sign next to "materials."
Right-click on any folder and select "Extract" to extract its contents to your hard drive. Every folder contains a different type of texture. For example, the "nature" folder contains textures which the game uses for trees and grass. Select a suitable folder on your hard drive and click on the "Extract" button.