This download looks like a map. You need to place it in your maps folder inside the Garry's Mod 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.
Flooky, Ahri, LOLEnigmatic, Derpydoge2016, xBobbaRaidz, Farris-Bueller, [C-G] |DANIEL, [C-G] Apéx ♥, Luminn~, KieraN The 2nd, KieraN The 2nd, ✪ Rob ✓, ℠ , Jimboblay(103 P250 Sand Dunes), roast, JGH, DatWizDoe, Sleeping | ItsTimmyB, ST_Vert, [BT15] The Suspect, Rasta, yo im iphone (FIN), Xx_Dark_PvP_Hacker_xX, Stara, FiB. Dean, Pulse, ♛ Rayne | 50run50 ツ, ezl has a little willy, Depetele, float, Dizzy, Sharp, Just James ™, Sections, Wheatley, Dylan Klebold, daandg1, AbodeRules, Icewine Rose, SainT, username, [TG] xXPro BakeR