Instructions
Open your GMod installation directory, found on your main hard drive in the "Program Files" folder.
Open the "sound" folder in the GMod directory. You should see a number of subfolders, such as "physics," "weapons" and "models."
Copy all the sound files from the "sounds" folder and place them in a new location on your computer.
Place the new sound files into the corresponding subfolder. For example, replace a weapon sound file with a new file in the "weapons" subfolder.
Load "Garry's Mod" to compile the new sound cache and use the sounds in game.