From the Super Mario Wiki
Game Guy is a Shy Guy from Mario Party 3 who sports a bow tie and a natural showman's attitude. He runs a game show-esque gambling event that players must play when they land on his space. The player can usually double their coin amounts or lose all of their coins, though brave players can risk their money for higher multiples. The games all revolve entirely around luck and usually the player's money is left up to fate. He owns and hosts one of his four mini-games when a player lands on his space, which they can be Game Guy's Lucky 7, Game Guy's Magic Boxes, Game Guy's Roulette or Game Guy's Sweet Surprise. Contestants can also call Game Guy by using the Lucky Charm without needing to go into his space.
The player can unlock the door where Game Guy is found, in the Game Guy Room, to be able to play his games at any time. Here, the player is given 10 coins and can bet any or all of the coins in his or her possession (up to 100). The goal is to reach 1,000 coins, but losing all of them means the game is over. Winning this challenge unlocks the rare minigame Mario's Puzzle Party Pro, a modified version of Mario's Puzzle Party.