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.
dsx3000, lucas-zoulou, joker128, [Earth Fleet] The Doctor, super shenron, elitedino, Redigit, v.adamjanos1990, martinkakos1, AytioGames, Heckelex, Heckelex, [Gallifrey Squad] Stardragoner, [Gallifrey Squad] Stardragoner, [Gallifrey Squad] Stardragoner, proli2000, GalvazimGX, nope, Surik_at, Doctorplay, Doctorplay, тѕсм.[SG-1] Teal'c