We've set the software properly and all (Traipse), but we can't find where to set which player is the gamemaster. How do you do that ?

You need to first create a room on the server. After that the person that creates the room is automatically the GM and new players will automatically be lurkers. For the GM to assign the role of Player, GM, or Lurker to any other they right click on a persons name and assign their role then enter the room password, if one is set.