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.
re15revisited, Tristwyn, Archmage Frostivious, bautista13angelica, Xeno, InvalidFish, [Sir]Sn4k3, Rpg Panda, [KOR]혼살InLv, l.maire c, Devilswarchild, Real Madrid, bauMi, O'Nesix, ✰VLRP✰ Vundult
Submitted by garrysmod.org