This download looks like a save. You need to place it in your saves 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.
dog man, WiggleMcPickle, aigars293, prokiller12, stryker, Abacadooblee, Dubovikvalentin, MathiasTheSkarv♥, [GC] LouieMc03, Adolfus Hitler, dorianfahenry, wot934.flo.934, iGodz, eraycan02, rifqi.syabani, ascenza.deluca, viniciusXD, deadhard10, pdev423, Master Kill, kazlauskienevida82, codd08608, azisking987, minhquandoa, hargitaibalint11, mmarcos008, FelipeRainbow, ghost15, [XOG]Jakey]JTB]ST], [XOG]Jakey]JTB]ST], Turtle Sloth, Desert Eagle, DOGE, Tvoy Batia, shad3004, Josh