Follow this video tutorial to fix your GMOD Texture Errors. If you prefer to read, you can follow the steps below.
Go to FragBoss.com
Go the the GMOD Textures Page
Download the CSS Textures
Download the CSS Maps (optional)
Extract the folder contained within the Zip File
Go to Steam
Right Click Garry's Mod, click Properties>Local Files>Browse Local Files
Open the "garrysmod" Folder, then open the "addons" folder
Drag and drop the CSS Game Content folder In the Addons folder
The process for the maps Is the same. You simply open the extracted CSS Maps folder, select everything, and drag It Into the maps folder In the Garry's Mod Folder.