The add-on on the Workshop that this borrows from no longer exists. My theory is that the creator of the skins -Rusty- had them taken down from the Workshop. This is why the textures are missing from this model.

But all is not lost! You can still get this pack from the developer himself on garrysmods.org!

~DOWNLOAD~
https://garrysmods.org/download/7385/zombie-apocalypse-skins

Once it's downloaded, just grab the file/folder and throw it in steamapps/common/Garry's Mod/garrysmod/addons/. That should do it.
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Side note, I am flattered that the lot of you love this model. That actually touches my heart, even though all I did was use Crowbar (or something) to import the model into Blender and did some light modifications.

Thank you for subscribing to this file, and I hope to do more in the future.

~MadMyke