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.
yajayrysik, bolamanteca, ramirezabraham318, thegammer107, joselita_sol, re15revisited, Pro Mantı, blood tiger, PootisPootisHoovy, Carrick, btt.7698, j8410140, Thunderbird, PotatoFarts957, night rain, 짱구ㅣFnatic, Doctor Who, BuffaloWings®, LilDevil38, Gaemi(게리인턴모집-, gerod.ford, Maro25, Preck-Kun