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.
Pupper, Liftÿ§$hiftÿ, Ford Z Zero, Autumn Fire, TW, Master Zexus, liamthecoolest2014, Antumarin, UltimateRobot8000, AwesomePinch, Zeldaboy14, RysMerana, Cave Johnson, Cave Johnson, Gree the Hatty, DonutJoe93, mzx987, =Stannis=, dadillstnator, applepielord333, kaydendg1, eagletomcat1, Sylveon, , BayPig (lego cunt), mls, eggysauce, link boy gamer SFM, Original Username | trade.tf, the MAGICAL hobo, edstringer175, edstringer175, √irtuαl√εlσcity, , , , , huge stupid fucking idiot, huge stupid fucking idiot, Icewine Rose, V92, thedoctor, thedoctor, Minembo, Minembo, Super Ratchet Limited, desupacito, Mister Prawn