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.
TPCU-1, Weiß, The Mighty Edwini, valentin.dhorme03, zigortfurkan, mr barron, foxythepiratefox2005201787, TutorialTV, el mata kakaneros 3000, re15revisited, Bzons, TerrorSoul, sergeev-yarik, mj01032003.mj, maggieiscool70, Bob Mikaz, Biahwtrooper, PepeTapia69, Alex, Alex, Alex, dovakiin, 「Brutal Taco」