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
Don't know where the Garry's Mod folder is or how to extract zip files? See more useful installation tips.
Fisktratt, 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, Biahwtrooper, ConnorJC, genarocairo, [TS]Luke, Turbosnail, elaymm4, ss, Misaki, Quizikcal, [AUT] Fuzi23, Seelookinz, Jelly, Cupcake_2.0, Hochmeister Rotneus, [A] TheRev. | Trouble