Any accepted player can be made a GM in that game, by clicking "Make a GM" on the player list on the game Details page. They have all the abilities the primary GM does except for changing game details, setting/removing other GMs, and closing the game.
Ah, ok. Cool. I wasn't too sure about this button. I thought it was if you gave up and wanted to pass the torch to someone else to run your game instead of you... Tooltips, maybe? :P