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.
LAY-LAYLOM66, dracarre, krystianwasiak51, KociaczekPL12, polak rebeliant, mineoski14, Sigalaeff0607, kriper, MatixZ, grzybek93bb, MineKlocys|PL|, The Safezone ( ͡° ͜ʖ ͡°) B, Çağlar, Çağlar, saveagosesavedan, prout, longmanh789, PSYCHO W34S3L, Dr.Christoff, maciej.witczak, Xgaming, kagan2004dilara, Pikachu, barteks16, SneakyBastard, Porter Justice, Bill Williamson, Roopekoo