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.
32canabiss30, Mr.Jack, Mr.Jack, Gormles, Braian_99, lbanarco331, No mercy for Traitors, O'Neil, Scout Of the 2nd friend, absza01, riybokon, riybokon, A.Shepard, igorcurak, [GmT]Gremlinzone(U.S Army), QTG坑, Didgit.Sergey, bonniefnaf, Mario89, Tailsfan2000, TOO-BEE!!, TOO-BEE!!, karim halawi, TheUnNamedGuy, kalfinio, DJ-sona, Lordgreenwho, Dᴇᴀᴛʜ Mᴀᴄʜɪɴᴇ, musti-gaming, ivo0126, kayser.1111souze, yurikentalens, Matt, [ZCC] Corporal Tediore, Jurisdiction, ZZchase, aalik1066, The last Gungie, ShamWow Salesman, Xen_12, pro donger, pim15, LeLennyFace , GRAND DAD, jw93859, Lanic, [The.Mr]Mr.RaynorScyFrogPan, auguk007, Sistine, CubicTuxedo +5 others