Gutbusters is a game that Grandpa has in Rodrick Rules. In a nutshell, this is how the game works: one player reads a card, and the others try not to laugh. Greg Heffley always beats Grandpa because he doesn't get the jokes. As for Rodrick, he always loses on purpose so he can laugh a mouthful of milk out onto Greg.
At one point at the end of the second book, Rodrick stole Greg's Diary and tried to make it into the bathroom. He would have made it in there is someone hadn't left Gutbusters sitting out. Its name is a possible parody of the 1984 movie "Ghostbusters".