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.
re15revisited, SirGibson, Cyclone Wolf, |Наташа Kовбойка⇝, Squeeze, Joe, absza01, Madric, Catmilk, Baconmaker, creative oyuncu[ASİL TR], BigWAT, wangshiping3507, xQuiCKx, Liam | WickerMan, Winter, NEO_EX, Reverend Jesse V92, B.A., Konec, Konec, [BB2] Faith, mikakater, S+eamChild, Syntax