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.
jenrygamer, masascuate666, alex_valenciana, juan2004kv, rainer.kroon18, polocata1970, matheussantanasantiago1.0, Schwieger, Schwieger, james lahey, final boss of mazda junkyard, Iron War Beast, JeffreyWoods28, Smokey, Smokey, Smokey, The Talibanter, Edward Elric, VOID, Gryphus, Alex, , Dao, ✯Radek