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.
Jojohnon, tjsimins, Kewin, fernando28_07, Satael, re15revisited, AdriusBR, TheRockeR, Yeah!!, Zombie Marilyne, Intruso, tothandraas03, David, Samurai Weeaboo, Darky, supr_player, rex21cortes-, A-Crow, [ARP] K., Xaphian, Pond_Rocket, Neicks, L0ng Boy, Omnis, Binary Space | SWFS, UncleSHREDDER, Asdaroth, jaygaming21, Krewl Sorrow, Tony Burback, It's Frenzy, Antagonist DC, [NOG] dat allahu ackboi, [E:NW] Hydro, codybrules, Jackie Saint, SEDEMSTO RAMPAGE