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.
kupcik15, dsx3000, ArabeskBörek, joker128, [Earth Fleet] The Doctor, Colin, Mystery_Culprit, triplemax666, Heckelex, ИЛАРИОН-ГЪЯРЯК, KonohaScorpio, stealthboy60, stealthboy60, dragutzu_stefy, тѕсм.[SG-1] Teal'c