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.
deadwolf516, Fire, Fire, Oskar, Kavat_Keeper666, www.BalaamNetworks.com, 2097597071, WoTaK, Sgt. Jelllybean3, SohcahtoaGaming™, ([(Bracket)]) | [CTPHL], §『㎖i_CRo ζoFt』§ | busy, Poot Lovato, Kirito, Rubythewaffle, Dark Savior, Tekmon_Xonic, Matthew, Pukki, vovanbl64, Simina-Cindy ಠಿ_ಠ, killerbunny, guardiant_t452, Zadix, Alex, Reck, ByB | Pantho, [HS] Smithy too turnt, [HS] Smithy too turnt, Resangen, Aftermath, Wepnet www.gangwarsrp.com, [WPC] Forrest Mark X, Emestar, SounDai, DaWhale, Matt, paraguay89, TheGandalf, BlazeITMeme #Atomic-Networks, EnderIsStix, EnderIsStix, EnderIsStix, Baksus, Luke, Sillirion, StealthPaw , code_gs, James, GravelSandwich +8 others