1st Method: Buy CSS
2nd Method
Download the textures.
https://www.dropbox.com/sh/85wtspjw8u48qf2/AACSo3AlRIuGNDPJf2Ks-wU4a?dl=0
unzip the textures.
Open File Explorer
Go to C:
click Program Files (x86)
then Steam
then steamapps
then common
then GarrysMod
Then garrysmod
Finnaly Addons
Put the unzipped texture file in the addons folder.
Run Garry's mod.