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.
MegaJonnyboy10™, TheFlaper, Spartan922, Vivikyu, shadowthehedgehog85, Feenie, wilcaredgardoguerrero, wilcaredgardoguerrero, WhiteWolf, Mia Stone, General Patton , bloodwolf1124, LinkPlaysGames, LolbitFnafGirl2017, yahel.rosado, Wolfboy1020, applepielord333, karkat0602, , DiligentSkink, ๖ۣۜJade, mondozappa, Wesley_1900, cadmonkey82, Snubbu, ThunderEdgeZ, skydome, Sander, Demonow thy Wife, okokoroko, sonic the 15, hotrod-1027, TheWolfHacker, 1stboyzzblack, Lord Ekul, Spunky Mei, rappy, rappy, bbenjamin1702, applepielord333, Saso, TOO-BEE!!, QuiveryMango, TwilightWolf, Sans_skelington, activatorulvoltzengan, hmbetawolf98, Zombie Graves, Kageyoshi Kuchiki, Kageyoshi Kuchiki +50 others