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.
lordkhaoz2008, machfachz, shakthu2711, Лохматый, libranoBrano, City17-Overwatch, Trashbang, Nksakos, werin990, =ΡΡ= 419
