This download looks like a lua file. You need to place it in your lua folder inside the Garry's Mod 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.
pccom.net, Mogiels, [ KARMA ], LazarHeX, D.va, Snow, GusLemon, NIVES, 10mouselord01, Auditor ♦, Leonardo2075, Souperior, yung, ★ YaYat™ Es Tetek, Nova, railfan9089, Zero, Ripmax, ><oHyy, HomeyGFunkOffACherryFruitLoop™, shadow dasal, Joey ヽ(⩾Ѡ⩽)ﾉ, Durge, Biffalo TheThird, Benx303, Ⓜⓟ➃ⓒⓞⓟ, YussefBR, AVCHonline, Azebal Agradável, Pootis, Lazer Face Chaser, Moose, O.T., apbeukema, Der Blonde, Der Blonde, Mms92, [Wolfire], (2142)General Reaper[FuG], Sir Not-Appearing-In-This-Film, y.landon, Lt Farrell, Lt Farrell, K3yboardWarri0r_011!3, Keshwoof, Keshwoof, Ryu-Gi #TeamPyro, Paul Bunyan, Sokkah Abdelkader, kylmking #army +1 others