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.
Mr. Vorpat, bartoszbartosz09, banettetejode.yt.2.0, AnAverageLisa, Spitz, 海綿寶寶, warcomer, vlad-slabko, Ashir_PL, PsyKoTMK, ☪.Space:Wolf, ADAM_HUN, Boogaclaw, turkufiliz, Doc, JesusHIMSELF, Warpath-the-drone swe, Warpath-the-drone swe, Warpath-the-drone swe, captsnow, HZMT-Jahalaa, Dis Dolphin Guy.., LazerSpy, LazerSpy