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.
mossesx, TheRiddler, chelpandre2002, madhuvanukuru, Dra19Rex98, Henry_bullock, tomasmattioli, iulian.bogles, laowoo, Stone, witherhunter101, gartj, mesikas007, Shornmonga, subaruiv15, yunaldo32, pink1911, kyshon25, [TR]DragonFire, gabirav, s0me1, Gerome, randrewf, sesilon-chileno, KAPITAN Marchewa, BLaMotte, ASH KATCHUP