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.
Alexi37, terrortwins, Pedruwolf, sssstepa452, Gryphin, comicsatwork, XypnisTe, kaerrickrc, Typhon, romaniaxfruitloops, zanofa88, [Savages] A Savagey Brand, tee-jay, hawkboy9000, KissfromFrance, riotbomberX, steelofbrotherhood, Vanman, Deshunai, osuruk-kokulubok, Millennium, TheRealWane, ✪ Tarix, ClarenceTheMailMan, Dark Zalivstok, Dark Zalivstok, Универ, Alexaro-Senpai, cigan, Stereotypical Mexican, Don't Fuck With Ma-ma..., Kronophonix, [SG] Bdcoll, Jesus, coachs20003, vPAyNv, Vault22Alan, Mahkar, jeremy.durkota, maulanacahyan3, Митяйкин, CandleJack, Rick and Morty, Rick and Morty, petukhovdmithry2018, Jeaner.Tran, UNSC Dawnforge 2142, Source SDK TV, Source SDK TV, in-flix +408 others