The Game Boy Horror tells Luigi in Luigi's Mansion where he is, what doors are locked, some stuff about boos, and it lets E. Gadd talk to Luigi over distances. Luigi uses it in Luigi's Mansion.
Mario & Luigi: Superstar Saga
The Game Boy Horror makes a small appearance in Mario & Luigi: Superstar Saga entitled the Game Boy Horror SP. In the game, Mario and Luigi must talk to E. Gadd at the Starbeans Café where he requests them to fix the coffee machines. Once every blend of coffee machine is fixed, they receive the Game Boy Horror SP, which makes enemies drop rare items.
The Game Boy Horror's appearance and name is based on the Game Boy Color.