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.
海綿寶寶, furiya6918, errollees, Rain_BOB_weapon_parkourmod_FUCK, skay364, ghi.ggygj, Wodahsthehedgehog, shadowIV, ezequielmorano, LeLennyFace , Fleet Admiral Tony, paopaoza1155, =PP= 419, ezracool1, TheAnonymous1997, omartag69