This download looks like an unpacked addon. You can either convert the files to an addon or merge the folders in the zip with the folders in your Garry's Mod folder.
You should end up with a directory structure similar to below:
This option makes it difficult to remove the content should you no longer want it. Merge any folders from the zip with the equivalent folders in your Garry's Mod folder.
Don't know where the Garry's Mod folder is or how to extract zip files? See more useful installation tips.
squierfield, Sluppy, OskarTheGoodMan, kehar aasheim, kehar aasheim, kehar aasheim, LOGANN THE BIG ICE CREAM, Defolt68, Tyron, Astamur Adleiba, shadowspace22, , alfaruh2007, Codez14, WP, Warthog, cmossi19, Genesis, GangstaLuigi2077, Mancave Caveman, AntiDanty, mr.vomito81, NeoTeXaS, R̷̄əđ_ŠFM & Ğämⲉs™, R̷̄əđ_ŠFM & Ğämⲉs™, xX LokS Xx, GrandAlf, PROJECT C-Subject, Donte, Donte, Donte, Piterox, പോസാർടെക് സ, Aoneex, Aoneex, Мурдила, Мурдила, hemid24, The Gunsilver, olibobs, Tôm, The Emperor, Sickness, Lautyxx, Труп Володя, svensis compiler server, Stupid Waluigi, iseeyounakedintheshower, VadimPlay459, bellito-bryan +267 others