[reed1103]

In this guide i will show you how to install custom skins for CS:S weapons. Its fairly easy, and a hobby of mine is making them.
Only reskins work in gmod (Skins that only replace the way the weapon looks, while not editing the sound or the model of the weapon. Custom models and sounds do NOT work in Garry's Mod.) 

1.) Download a skin. The most popular site is Gamebanana.com, but you can get one from another site that has CS:S skins. 

2.) Open the .rar or .zip that the skin comes in. (Make sure you got only a reskin).

3.) Locate the materials folder. 

4.) Drag the materials folder into your garrysmod folder. ( C:// Program Files x86/steam/steamapps/common/garrysmod/garrysmod/)

5.) When it asks if you want to replace the current materials folder that was already in there, hit "yes". (It won't acutally replace the current one, it'll just merge the two.)

6.) Finished. Enjoy! (I recommend deleting the "_ref.vtf" file that comes with the skin, mainly because it makes the skin look super shiny and almost unviewable (on hdr and bright maps). In CSS, though, these _ref.vtf files make the skin look better, because CS:S has different light graphics.

EDIT: Some different models and sounds work with gmod, but not all of them.