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.
[bot909] . ..◥◣ ◢◤....., JimOverThere, CIoud of the Rainbow Skittles, Adolf Hitler, MaximCreative, failgater, Edward Elric, [LAU] Jake, Kuroda DarkStar, Special subject, xerxes™*|kbs-dsj_pb|*, Thou, Phulax, boblealeonard, LazerSpy, ✯Radek, ✯Radek