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.
piggie203, Lily, andrei_alin2000, W-G [John Max] AMWDI1, Yagballs, saucyballsenjoya, Some Kiddo, thermayder, 「RCG」Bloody Spy, pavarisanew03, djliebe33, 콧코, Preglas25, Pandelavieja, Alex, pizdddetts, MLAATRFan2004, TismaProd, jedsamson102, ⭐⭐⭐ Sebastian, jrod, raoul duke, Tim B, FunnyStarRunner