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.
tanyusha-89, riverciandreya, christian.hexum, |AnImE|Kitt3h, JahiMiddlton, vakarianbg, Reverse, camejjones03, camejjones03, Tim_Jedd, zennat.3195, william.winder03, bonnie.gta, [DFS] SpycrabHtoo, mrjungles, szegacs, rodrigo.alvarez.bahia, Doktor haus, Denisb96, Dad, Borisov, ECHEiON