Instructions
Download the skin, texture, item or model that you wish to add to "GMOD."
Open the downloaded file, and extract the contents somewhere on your hard drive.
Open up the "garrysmod" file on your computer, located where you installed the game on your hard drive. Locate the "models" folder in the "garrysmod" directory, or if there is not one, create a folder with that name.
Put the contents of the downloaded file in the "models" folder.
Copy the contents of the "Addons" folder in to the "garrysmod," replacing any files that have the same name.
Load the game and select the model that you just placed in the "garrysmod" folder from the in-game menu.