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.
zanofa88, koliamezinskij, ĞîĐŘǾ_PǾŇÎČ, J4nk5004 | gamekit.com, NastyPorcupine, JordynReilly27, thepxaychannel, DECOY panda1, Nasca, Archiek1, Spartacus, ilezhnin, Azevm | Virtual RolePlay, RUSSIAN_CRAZY_DOG, Sonic Alchemy, Where'sTheFairWiener (OFFLINE)