This download looks like an addon. You need to extract the downloaded zip and put the addon folder in your Garry's Mod addons 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.
Ace of Kings™, over 9000 hours left, deserteagle1446(Shag), I am Plen, Falsii™, MedEvilSourceror, Fat Lobster, dhgfgfg6, ๊K๊a๊n๊k๊a๊, Lucky Luck, Lucky Luck, POLE, POLE, Zigzagoon (cyka блять), [MG] AlertIowa1234, A.R.C.A.D.E. JEFF