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.
jakubszczypkowski7, Aryia, NathanDarkson984, Falk | Falkitop#46, Unusual, Caz, SøMuɔh, 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, R.I.C.O, R.I.C.O, tr271031, TheFruitbat, james brent, BigBootyLuna, BigBootyLuna, varzol75, Redwombat8432, Bishop, TehCanadianSpartan, a sad rat, Husck, AMGWINS, Weirdness30, Alarm, 3.14K CHU, lanciarazzi_41
