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.
smash_bros_07, Karljorgen, Deokaeth, Kazooaids, Bob Le Marteau, [DG] TheMexican27, Greek Guy, xneonninja420x, |CRP| Dragine, ☜❶☞Predator☜❶☞, ChinaMann, Thomas10152000, garrsmode934, Warhammer, ⓂⒶXⓘⓂ, White Palace, NercoLive, ▲[OG]illuminati▲, [!?] Gamizard, VintoreZZ