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.
RiyoShi, TropTeps, [EG] John, Squidler, [MG] Alex Marchand, [MG] Alex Marchand, U-Mad, Wicked, >>>[!!! Mr_D !!!]<<<, TwSt ACTIONS, Monz, markusmarkusz, [Ｔ]ｒｕ [Ｌ]ｅｍａｎｏ, SeanBHDTV, Mi40, HuskyPuggy, |Z| Max, Scrat, |Ďк|2Mαŧιαѕ, [NRG] Game Over