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, garrysmod1300, mesut_ozil_999, falger, some faggot, gh3, Dott2000, busermanov5, Korneliusz, Khangcubu123, TERKA G, GaboRecords, Lieutenant Archer, burakv2008, Zulus, Captain Quantum, ahsan.umar01, FATTYCJ, jwnovosel, Kurumi, Robotnik, =PP= 419, [CZ]GAWDO11[CZ], NotWaitMe675, Brony Sniper, Soldier