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.
pavelstepan-ucet, killer2004 HUN, neprest87, yamahonkawazuki, Mr.Blaziken, pedro75aderezo, FLX, minouthabet, nakkalokesh966, lorenzbryan, bentadil610, tuvshee_23, petar_985, guesplane37, seriousrobert, reinisobuhovics, Gevatter Tod, (141) John Freeman, Riha.Jarous, TheGamingCharizard, ivoyer2, Swiggy swoogy, msatour51