Garry’s Mod is a sandbox mod for the Source Engine. Unlike normal games there aren't any predefined aims or goals. Players are given tools and are left to entertain themselves. The player is in a world where objects can be created and destroyed on demand. Where people can be created and destroyed with the click of a mouse. A world made for him. A world he made.
They just came from a gray swamp... When we finally got to the city, there was nothing left but gray mud.. a lot of gray mud. The addon includes a model for players and NPC.
Open the RAR file, there will be a Monster Boys folder (PM & NPC), then extract it in to Garrys Mod\
Example : C:\Program Files\Garrys Mod\garrysmod\ addons