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.
joker128, animations.jeunesse.rottes, Col. John Sheppard, RRT CEO | Picard, ❤ C R E O N I X ❤, Heckelex, [Gallifrey Squad] Stardragoner, Subflixx, [ScifiAdv][XCO]Cheker2500, Madshinypaper., Reverend V92, Breeko, Brucektrain CityRP Gaming