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.
wwgaelori, Tanukii, ekkivox, ★ ZEDREEY ★, miguellemos564, abir89525, Eric, It'z The Greatest Lad 21, Overthinker, Zeol UwU trop baka, ryxs101072540.ando, 3327-NT, Johnnybob5, Doofenshmirtz [RADIO], Ʉ ₦Ø₮ ₥Ɇ, warframeninjasword, muhammedtahaerdas.06, Agustino, alan12_34, [SFL]JhonF, Goose | RN, Pinλplle, Matt Mattay, ☺DazaYCHIK♥, Dotheboogey678, IllI, IllI, Otaku_Pro_Gamer, hw.cc, Miki :D, Player-0, Player-0, Player-0, Player-0, Aliskair47 X, DEAD, Nellored �, bulwhyf, that dude from serbia strong, fidel castro murderizing someone, Maxouuuuu