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.
☢Stalker39☢, bashir, leonberna, [Retro Luminatas], Macthelost, BlackHawkTP | R.I.P PC, T800-Model 101, T800-Model 101, کǺпd€Я, BDR(BlackDenRus), turkufiliz, Dr. Savage, NightAngel, YoRHa 2B, Shalashaska, aalik1066, Svensis, UncleSpai, mcLAREN, mcLAREN, Respite, Icewine Rose, Mike The Comrade, Reverend Jesse V92, Reverend Jesse V92, damir.balgazin574, bm killer10 ☠ ⚠⚠⚠, Hds46, borisbritwa91, Ŷashirmare, TeamKiller