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.
egor.pestoyakov.052, Koopashell TMK, Lord TL [Hun] ☭, ArsavoliNiu, Warpath-the-drone swe, Warpath-the-drone swe, PC1073, auguk007, Minimole the Lorax, Blaze the Pegasus, TEHDIAMONTRULL, fawfo, Di`Kut, Dis Dolphin Guy.., Soviet Comrade, SevaGlitch