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.
Rain_BOB_weapon_parkourmod_FUCK, gorkem_agar, ~~Snorgan~~, SirTalis, ☣ Vexer Wor ☣, ☣ Vexer Wor ☣, IssKuu, Commander Toros (KPAvids), dumbbatman, Warpath-the-drone swe, jirapatpu55, theundergamer, theundergamer, George 08, George 08, stabkamay, LazerSpy