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.
Bad Bunny, Northy!, Jason, Linkl, SWED_CODER, Ellie, dufa, Rerabox, syno04, xXx_DoomGuy_xXx, .vurk, WillH, Accorel, ★ M Arisato ★, Jagged, ★〖C〗〖R〗〖4〗★, Jeff, OP_rah, DrankMySip, uck foff, Fear_Dragon I hyplex.co, Barthez GoldGoal, Kevlar, jojoreen, BlueHairedNinja, WolfG, Lord Splash, sans, Ccsdkb, Salad <3, , Dogsailor, ҈Ӑ Ķĭľľęõ ҈ ҈Ӑ, ҈Ӑ Ķĭľľęõ ҈ ҈Ӑ, bbenjamin1702, M A L W A R Σ ツ, So Doubtful, So Doubtful, Slighterer, Slighterer, TOO-BEE!!, TOO-BEE!!, TOO-BEE!!, ☠☢☣cole12169☣☢☠, [PV] Sinay59999, Draft, pyro god, Agent Carolina, seif1, Metropwn38 +175 others