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.
axelnievas22, dnazarej, dnazarej, nakkalokesh966, DECOY panda1, [F2P] Rajderr, eddyochoa2004, yoru1708km, Mystery man, DecadeFury567.exe, Alarm, matheusmelloliveira, SANTI48, Xxcarluch13xX[MEX], yasser, TriggerWinkle [SFM Posters], funeznavas.cristhian23, =PP= 419