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.
seb.maczak, arina_dymbrenova, (2018) edenarmar907, k1tsun3kkyi, Braian_99, v.politick, Fox 666, SKITZOSYKO, Justin Jeans, Ariko-chan, Not Big Soup Rice :3, pako80, Legan, decadent_uk100, hanoka2034, jctaylor2025, alexmdivani, ivorleft4dead2, trencox7, thedork4talkshow, Teşekkür Ederim, Umbraevus, R1p0ff, drumstick1113, WolfMast3rMike 205, jwnovosel, I Had Sex With The Pope, marvel, marvel, marvel, marvel