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.
mossesx, javierlarreynaga, Acecool, Yucky Fox, Kyozhou, ๖ۣSlow star™, Nopeful, Edmond, Asriel, Barney, SkryZZ @lan, TipsyChica 3.0 #VLADIMIR, Scorch, Keshwoof, Vulfski, Viper, Velemeister, ™ Magicsfire ® ۞♫, Fluffy Eevee, フラックス, フラックス