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.
Lala San, gaming chair, mein gz, Kai, jakubszczypkowski7, Aryia, NathanDarkson984, Falk | Falkitop#46, Unusual, Caz, SøMuch, Magical fern hat, G2 Team KennyS, justbe16, jeanellebarrientos, mossesx, NicholasJudy456, dan.oconnor666, Thequagmire, bluecatcharley2006, bluecatcharley2006, wet pancakes, gabybarrocos, gabybarrocos, S_Eria♬, danilstarbird, Hellanico | Farmskins.com, [email protected], [email protected], tr271031, TheFruitbat, jamesbrent, BigBootyLuna, BigBootyLuna, varzol75, Redwombat8432, Bishop, TehCanadianSpartan, Latimer, Husck, AMGWINS, Weirdness30, Alarm, I am painting, 3.14K CHU, daniel_fs0