This download looks like a save. You need to place it in your saves 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.
♛K I L L E R♛, lindsey8228, Tevuori, babaarda2005, storm, freebelion, [OHAPIC] CptDashie, Polkan1221 | TRIGON.IM, -[Izakian Private]- Michael M, alvajogaw, [SGE-A] Blackdead, maksymiliankryger, DanskeCrimeRider * CM-NW, [CRP] SilverSky, jimmy, stealthboy60, Joe Biden