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zs_infected_square_v1

Either buy cs source on steam, install, and then mount it in gmod (bottom right, near where you select gamemodes) or google it and download it as a .gma or an archive from your website of choice. Installation instructions are usually also there, and may be specific to the resource you download it from
If i were you, i'd rather buy cs source, since in my experience a lot of websites that offer content insert their own resources (like changing the default spray) to promote themselves. Also they don't include maps, which you might want to play
A lot of maps in gmod, mainly ZS and deathrun, are ported from cs source, and the game doesn't have cs source textures, models or sounds, so they fall back to checkered textures and errors

yea i dont know where to get that at

CSS content most likely

it says error and its missing textures what mods do i need