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.
GOLLOUMMMMMMMM ⚡, re15revisited, A Combine Elite, FrozenSquid, Scamming Computer, shrek ( ͡° ͜ʖ ͡°), andreitagsip, ブランドン, Jayzoh, Biahwtrooper, Nekogami, UncleSHREDDER, ĦydʀαPhøeŋix, JynxZ, Mister Fael, The Raven, AWACS Space Eeyore, Mullins the Great, w a z z, Spastik