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.
GrosserDulli_, Ako, Clownfiesta, Clownfiesta, Timetorun, Prime, ReVilosHD DER BOSS, NightxGamer, re15revisited, Amori, [URP] Jonas Drain, ape, ImGreg, Timbo, T0FF, Jarkar, qaisjp, Sergey_26rus, KentiSHika, S3RP3NT, Shuko, TheRam, Mr. Stevens, Tedori, kenny11161, Swope™