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.
piplup_the_panda, AGRESSOR, rmn1981, hugounivers6 rm, Warhammer, Mamaluigi2056, Night Ghost, ikorih-7oct, [email protected], [email protected], Zapper770, Sneaky Snake, Rising Star, kutikut4, a1_ahmed20, andy.chalupka, METAL-HEAD, ░▒▓HoverCat 0768▓▒░, IC, Cybernetic_Hamster_1031, Lettuce Cat, TheWalruss, Danielski914(>-<), NiallBTboy, RyanTheNerd, Edmond, 할 - 플랜, Xxx360scopeXxx, aalik1066, Kebab Warlord, faulkinpunch, Ryu, Warpath-the-drone swe, ACR_Hutch, carlos_zerda, [BB2] Faith, [BB2] Faith, FisceralV|NSA, Shadow_Knight, Puke Hammer, The Unlucky Medic, .:|WkLamHenry|:., andrewdbuilder