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.
Alpha1123, ImNothingSpecial, The Frenzy Gamer, SFZ Gaming, HappyTheBurger, stagkiller121, ExoTic_CS.Money, Chaii, Wandering Overlord, Young Chip(SoundCloud Rapper), DeltaRex10, I will wreck your assets, TheGamer101, tothandraas03, victor_9212, kmdnsteph21, Jezzabell, AC❤POOL, Soldier, DiamondGB, 6twister4, twitch.tv/Ryder765, Kobra, -[FF]- Red Ram (赤い羊), EginKaman, Soul Eat, @ KumaNeko-Sama @, Avoid, X dude Awesome, Monty