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.
Vhoxel, [=SC=] Gewaechs, mindy, Bunneh, Crankerious, ✪ spoon, Xeno, InvalidFish, DuskoOphra, Dan, josh_da_ninja, Yuuki, [KOR]혼살InLv, ]HeLL[ JayJay, fartsmellovly, Blue, Zicke Kamikaze, Mr.ROFLMAN, O'Nesix, meharryp