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.
UNKNOWN, Vinyl Scratch, [RG] Terry #VapeNation, »[AG]Scriby (ᶜᶦˢᵘᴹ), razzan, hewlet, hewlet, hewlet, hewlet, dhgfgfg6, [GGG] BageL, [GGG] BageL, DarknessNinjaBR, _0Nekit0_, sandiegoklassik, sandiegoklassik, Dookin Foof Lord, Soggy Bread, killer_team, MilSchwitz, Lenin Cat, Bwillie0, Archy Bot, Archy Bot, Archy Bot, LeoDoesStuff, Reverend Jesse V92, kyle.stokes321, Hoplite, Hoplite, DaRkWoRlD BTC4KEYS⸳COM, Captain Phasma, ByPoLaT [TR], I put on my robe and wizard hat.