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.
Blowshot, Sephiroth, Bunneh, sarzfull001, andreas-taylor36, daddy why wont you love me, fruit, ivan.andradeeqa11c, marcnathanieldvillaluz, ✪ spoon, ZippoFuegote, F_N_Play, Professor Plum, Xeno, Utkarsh Singh, fuNnq Surviver #41, EpikMythic, web.lbcrafter, TheDamdot, basmsarsur66, laughing bomb, decadent_uk100, ElNegro, Floki, L u x o r.- ツ, Mechatronik, Napstablook, Niels, Nadie, [KOR]혼살, Bwae, KiroFoxyScout, Rice, [★TBOB★]Doctor Y2J, Charmz, ZZSark, glen_loo, Top Doge, AnimeDreamer91, Jesus Christ, La Zick De La Kamikaze, Kat, MightyS, =RGS= Mr.ROFLMAN™, ConFused, peytony9