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.
Vtech just kicked in yo, Andr3w, zskvor38, danilavirigin36, puypuy57, Unelith, -XV Shock Down, micmac253, struct67, electrobyt1337, RailFan9089, rileyjay_5, AE | Contrast, sam1995agent, sam1995agent, sam1995agent, XTRON[ESP], Erwin Schrödinger, daehwan0963, Vongola El Brecho, thompson, Ramzee, mihajloweed, emilianofranzot, Wullen, Belamir, londonoruedak, ImmortanJoe, Aroxlist, tadashi, SuperSaiyajinGodVegeta, beedyCZ, NorthMustang, petersmail, bjones4573, PinGViiN, letop06, cobalT, PL4$M4, Alarm, danekyro, danekyro, Mito, Pr0Face40, Pr0Face40, mmarcos008, Chris Almere, Sly, florinmoldovanhas, [NG] M4dhead +22 others