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.
syN_ost, hassanebraheem2010, kovtanstas, jordan08tho, zelia.vvol, kitlucky88, °, gamehag.com rick may, Mr.Nik, sharbannadarajan, seidrakhimovich, seidrakhimovich, seidrakhimovich, jcicache, isaaclobo449, pavelumrixinmail, elpepecorrewachiinn, elpepecorrewachiinn, rajputmadhav572, Галушкэ, mikolajkwiatek411, ressel455, maks.vlasov.16, edgarsgrivins, Some random Bosnian guy, Suzuki423, samikboy, georgi_jakimov85, lorenzolualdi92, com.antilles, 2593282299, Akiiyaamaa, Efsanevi Altın Şovalye, preslavkrystev, fk134679852, francismototo, oulhassimouhamadfaycal, TAUMMEUX, SheyRo, nassordhiab742, FANTAGAMES, SCOUT, michalwolf, допрыгался TкофеT, Dr. Faust, Am7xen, Dark Dedpool, reaper71, K, K +1116 others