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.
karlogutierrezmusic, jorgeaugustobiziu06, adrianjudith12, tort00, RealTNTER, 漢 H A A J E X O O O N 漢, makytitan023, niki_wasp, Combine™, jakeboyharris, jakeboyharris, =:._J040S1LV4_.:=, Shadow, chulong2004, nicolasvieirabonfim