Gamemaster is a platform gamemode for Gmod 10, with a twist - you are able to play as the computer (the Gamemaster), trying to prevent the players from completing the level.
As the Gamemaster, you can place enemies, walls, and so on, while the other human players attempt to complete the level. If all the players die, you win. The Gamemaster will stay one until one of the players manage(s) to win. Then, a random player will get to be the Gamemaster. You are allocated, in each level, a given selection of enemies, obstacles, and such, which can be placed anywhere offscreen (so the player can't see them being placed).