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.
stefant19901, |"DN"|Rage|, Colonel Vyse [Kenji], alex10, LAY-LAYLOM66, LG | IllusionJM, ya.theskeleton2016, Lenny, thegammer107, Edward, HELL, HELL, Joohnny_06, GORYGIRL1996, Mister Cross, Mr.KTO-TO, eDp! Scumb4g, Chimp, robbed by a chicken, robbed by a chicken, robbed by a chicken, Shelest_, ghost3215, [email protected], [email protected], Boogaclaw, AdvisoryCookie, Cor Angars, 178086057, salo_S94, FarhanHQ™, Michael "Noice" Rosen, jthmsonicfan, xXx_MLGNoscoper_xXx ( ͡° ͜ʖ ͡°), L4D2Fan13 (Birthday in 5 Days), Forg(R)otten, Forg(R)otten, nostalgia. #shitcoder, Warpath-the-drone swe, Warpath-the-drone swe, selling servers lukefulleylove4, The Pancakeman 23, jEYEM, Higherson | Roadkill.com, Stukakukaukaukakukamodi, meharryp, Rexxor / DigitalDeltaGaming.net, draconica, draconica, PeculiarSentinel