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, namesch, Squeeze, 2420472579, andretheshotgun, Todoo189, john_f_kennedy3, Niko1U, LMR_1, ไอเจ้าเน็ส, Nutmegger, Sheev (The Senate) Palpatine, potocki01, Doomootron, Doomootron, Doomootron, fabif1997, ༺ۣۜSweGasmaskGuy༻, Mr.Pro