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.
Soap, _ProTein_, Chewable_Tablets, ◢ k r i s ◤, venokcorsa, [blue]Niotic, Vorius, Keith Keiser, Shaidy K, YourNan'sLeftBollock, RetroWeb, berryarg, [IG]ScrubbyTSD, snokedogg911, PsychoKoopa, лиззка, bashir, don glover, BigMan983, Mr.Zombie, kgergo5800, archicastor1, porakorn76, Starship Trooper [Rec , A57RO, Lowblood, selcukkuscu22, Russian Empire, Mouloud Gaming, OSITO (TED), Lamb, Difficulty Tweak, Fernando77210, senselessArtist, randomguy376 farmskins.com, FIRΣ STOЯM, *******.gabenewell, fukalotivana150, unknownblanked, Tyr, Samurai-Jack, Fabio, Spaz-E, ╭∩╮█ Mr_(uçar) █╭∩╮, [SSG] 2ndLt L.Payne [SC], inthefort, Sauerking, God Jr., Col.0wneD, lucahenner29 +954 others