Adding skins to gmod

I need this skin in gmod (to replace the medic skin), i have no idea how to do it, so can someone please tell me how?

Install it like you would any other skin, except in the Gmod directory.

i think he means he doesnt know how tp put in a skin

if so all you do is put in the materials and models folders in C:\Program files\Steam\Steamapps(your user name here)\garrysmod\garrysmod
and say yes to all