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.
al.anwar, neiltamboy, danielflupaixao, matroxzHD, DVD_Screen, MrSaturn, jose.hermoarce, samueldylon161, piotr.muranski97, bjv, loganplayz1011, dzbear, romyc1984a, Dumkal_33, sebasmoutt, 맹, amierziz090, Samuel_64BR, gustavogabrielrc, gustavogabrielrc, HDMONEY, awenza321, netro9588, J-Lova☁, allanjohnson355, fatboob3982, Saman.exe, dimitrijevuckovic026, Quadrazid, GÖKTUĞ ŞURAZ X PRO, Total Gentleman, hi, donot.not11, meow, michael, michael, The Star-Crew 5, Black Humor221-228, LambdaGuy721, Cheruno, demirtas-sam, deanudays, ahmed, the soldiers defy movement, mici99, eymen445566778899, The Original pootisBird, GasMaskCitizen, PainOverHeaven, golden12912.0 +2177 others