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.
nyarl, PerSPot, I Fix You, Brutus, DR.PEPE, carcx01, soygamerloquendo711, Valkyrius, pnaxo2008, ttebbs916, Cetus-The-Cyndaquil, #Lucky, zRDM, Soup, Cabada783, The_Hentai_Senpai, Memelord Spence, Dugger, chrsmca1, FabulouslyCute, dani_dani2719, ZombieWarsSMT YT, Wulfie52, TK, austincolton1314, Fricabis, Mr Soarin, Mr Soarin, MrChoCho, BountyHunter29, Im A Masokis, Fox77itax, Braian_99, Braian_99, ZapokeTheBoss, Thehuntsman, Steven The Screwed, Yeppes, TheKevinator, KoondiX, ThatPedoNextDoor, hgraboid, Zer0 Amnesty, FrostedCakez, Xenable, xd, C.Z.R, Halbard, MachineGunLee197, Little Miss Squidgy +47 others