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.
Hexxzz, battlecats14508, F-A-s, joosep16, stas.astakhov.2002, Braian_99, mustafaev.orhan2016, ManqkaBG, bBb_Bcrublord_bBb, alvarsskn, rmn1981, applepielord333, DarkF0x, MrStarischoc, Riebulka, busermanov5, Бетховен.mp3, About 7 People, keReaker :3, ZeroFactory, MasterTree007, XxXMidgitmanXxX, gog1701, TheWelshGamer, derrysneddon02, Uros Uki