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.
firstname.lastname@example.org, JembutGoreng, heatherardoin, ☭ Lord TL ☭, (ISIG) Hailbreaker, Nattamon_boss, Korbenn, theundergamer, surfer171, lonely19682005, anicity12350, peytony9, Dis Dolphin Guy.., AllytheWolffy98, Picklebond, MikotoNui, PeculiarPsychopath