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.
sampistola, risoskol, Candy_the_cat2, yutsamon, Aquila Ursus, Delsemon, MADWOLF, MADWOLF, Riha.Jarous, grantdg1989, deidarasen666-2, Halo44327, Gagnetar, Super Ratchet Limited, Jd, [GMTH] GmodTH
