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.
코끼리, Papa Smurf, Quick, Hail Jxel, Triggered, [KOR]토픽, The Combine Soldier, Space Sans REVENGE TEAM, Minekohle, Drawcris, [TG] Wren Adler Games | NHS, OG ACID CSGO, KiiZyx, Qumusabel, [Dutch] HarmlessEppo, 히스, 치킨이, Wiz Z zLe, 세니, еврей в ½, 똘끼, goldenrabbit, 순수찌, KR성빈, Zemes, Eleven, SpeedWay, Baguette, kor_dora, 제비탕수육, JellyQ / hunter, 강비, popovdani2004, Sk8erace1, Cramyaaa, |-=ДикалирНый=-|, CsG | MrHicK #bluvos, Timelessblue22, jyc7848, 비지닐스, ZUERO | Walter-_-Branco, ☣ 어리석군 ☣, [CHIEN]Bowanga, Ghandi, 나투라, Hampy, super3467, SomeOne, TazManiia, I Kuma I +71 others