Hi today i gonna show you how to change your background first of all you need to go in
C:\Program files (x86)\Steam\Steamapps\common\GarrysMod\garrysmod\background

Now take an image you like me i take this :3
be sure to take you'r native resolution or it will a bunch of pixels

and here we go your new background is in your game enjoy and bye ;)

and if you not happy i come to you>:c