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.
MTYMAC, [Fondateur]*PanamaRP*Micka, abuj090105, Not Alex, Mario, Pandamonial, mikesbell78, Draluigifan222, jeniffer_jsr, jeniffer_jsr, Bongus, jdramirezelmo, TheGuyYouWantToBe, Sokkah Abdelkader, Sokkah Abdelkader