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.
niandolo_92, gginmypeepee, Hitler, klingmann-juergen, Tio SVD Disponível no XOne, bertisis, Manchester 25, Gretto, peaw._2012, masterlagartox, FUCKING SILVER, shody94, norbi32, DogTheLog, ytp123, Gronkey Kong, Robbevsr YOU NUTTN