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.
cbothma, qazwsxedcfg555, escolargh360, re15revisited, namesch, Squeeze, DustNeck, 2420472579, andretheshotgun, Todoo189, john_f_kennedy3, Niko1U, AC❤POOL, LMR_1, I am your sarge!!!, Nutmegger, Phreek, potocki01, Ezz Pulchrum, Ezz Pulchrum, Ezz Pulchrum, fabif1997, ༺ۣۜSweGasmaskGuy༻, Mr.Pro