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.
re15revisited, krpndeth, Bartek, Coincidental, greybatter97, Doktor haus, Ω² Big Smoke - kenneth, Britboy, horse with a broom in its mouth, Crazy ol' Cheeseburgah
Submitted by garrysmod.org