This download looks like an addon. You need to extract the downloaded zip and put the addon folder in your Garry's Mod addons folder. You should end up with a directory structure similar to below:
Don't know where the Garry's Mod folder is or how to extract zip files? See more useful installation tips.
SPRINGTRAP420, (Combine Army) HATAWA_RUS_86, (Combine Army) HATAWA_RUS_86, QTG坑, vmskustom, Folder X, RAMPAGE, BigWAT, Matt, Porter Justice, UNFORFOZIS, UNFORFOZIS, TheGreatBrandino, idfix67, Willymont, Rose Nomin' Duke the Bad Dragon~, oliwierchebbi