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.
```````````````````, sfodd, This is potato?, [SPMC] HOpt. Bvnditi, Red Jack, lagrom12, Papa Emaz, Mother, Trailblazer, Leamas, m.iovenko, eagle3zio916, ✠Conte_Alfonso_II_Gonzaga✠, Someguy2132, ShoyuAGoodTime, Answeeder, Stressed Operator, JohnSheppard44, mossesx, liew, Ultrawins005, Mr.G, Andrewq, JD, Сука Блять, archicastor1, glow, MediaFire NO.1, tobiassilva110, bashka, ◘ [ TesBoi ] ◘, ◘ [ TesBoi ] ◘, ◘ [ TesBoi ] ◘, Reinboa, Pew Pew, A4MADxKING, undefinedscott, Lemon Juice, fightherot, spaghattosnoodles97, spaghattosnoodles97, spaghattosnoodles97, 5 Cent Pete, Crimson, SarcasticSpastic, Superсat, Reb733, Masterak555, Babadaeus, dota_7396 +184 others