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.
tirosgamingyt, Mike Wazowski | HomebrewGaming, (DEB) Scares70rm, Gaben's True Form, Chimera, Derpy | ZENGAMING.NET, GrenerLetsPlay, KX | Taiwan Jeff, RileeZ, Swiss-bones, Coolcat, ADiBOU., Boo Boo Kitty Fuck, The Stig's Teenage Mom, Mr.Socke ♥ Scorpio, faso.wav, Wht Blueberry, ///TaxWeek™///, Master J, Chase Hall, BobbiPlay, TELLA22, My Dime Is Up, Jay, Desk, |SG|Chris-Skulls-Gaming, Isaac, [GER] FaceKirmes™