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.
unityazwt.g6, DarkZhadow1177, albertromeroramos#8906, Luca12, cirvis4999, Ossuors, mikejibjib, MonsterKiller, yngxta, dimitrijevuckovic026, CARLITOS NOPE F2P YT, Darki StatTrack™, HELLO_PIZZZA, nojux000, 1530076976