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.
luca, |FreedomRP| Taube, PEACEKEEPER70, -(G-C)- Djeamym, IamDoby, iariesi, O pistoleiro, Blake - Owner Of Galaxy City, prokiller12, greygoose911, AssaYer | dogry.pl, GunnerCWB | Hyplex.co, ๖ۣۜDu$tin, Кариша Сычева, ChinaMann, [email protected], Neal Bridoux, |CRP| Ghost, Harambe, voltized, aleksansander, Blackracer10012041, vSync, Spades, Packert, proxy.1, Derp, Aɴᴛʜᴏɴʏ Fᴜʟʟᴇʀ | SᴛᴀʀGᴀᴍɪɴɢ.ᴍʟ, Stefann!, HAZAR, Not known, thexkey | trade.tf | Zeno, General Resh, TrixalOfDeath|Hyplex.co|, Chief Keef, Desert Eagle, KidComet, Glenn(trading tf2 items), CaLeb, CaLeb, banana duck