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.
Tonton 41, mafioz367, _FiRe_FoX_, ejzp_06, Offspring, mike.f040817, GLOBAL NOVA ELITE, RedPowerCraft, NanoLyt, Windziarz, aziz464, -Criss_Cross-, antoine9599, mommix2020, zaferayyildiz55, Барбариски:З, szymonstolicki, thompson, facundokapo44, zfadarkness, IDKWTF, [CG]ChAos, Alarm, Hakirito-Sama, SteezeMcQueen, |DG| SaintlyDemonic, Rewolution[PL], Sa_Nya, Frost1551, Theo Dwayne (Lacraps63), ⿴⿻⿲⿹⿸⿴⿺⿴⿻⿲⿹⿸⿴⿺⿺⿺⿴⿴⿺⿴