This download looks like a map. You need to place it in your maps folder inside the Garry's Mod folder.
You should end up with a directory structure similar to below
x_vadimic6_x, petrifiedcow, Alien, re15revisited, re15revisited, Royal, Reudinger, rrodermel, Thewisplay_003, ❤CookiNut❤, CommanderPander, WotPot, The Best man Ever, Vakama112, Jeffmoncada14, M-Ninja FoX.ReVo, MR.Arrow, Zaccy, Yuuki Rei, PaulaPaulaDeenYall, Chungy, [CdC]Alexaldin, sexyGacruxTheWolf, cmchapin, <a>Turtle</a>, [GFL] Church, CrazyRussianHaxor, [GFL] Kitty Kat, ConnorJC, genarocairo, [TS]Luke, Turbosnail, elaymm4, ss, Misaki, Quizikcal, [AUT] Fuzi23, Seelookinz, Jelly, Cupcake_2.0, [A] TheRev. | Trouble