The Goosy Grow 4000 is an invention by Professor Poopypants that later got shrunk by its creator alongside with the small Captain Underpants. It later was owned by George and Harold with the Shrinky Pig 2000 then made an appearance in book 8 but it was now 1 inch, due to the fact that in episode four, the Goosy Grow was shrunken with Captain Underpants.
Professor Poopypants makes this invention with the Shrinky Pig 2000. He later uses it to take over the planet when he enlarges his Gerbil Jogger 2000. But when Captain Underpants attempts to steal it, it gets shrunken by Professor Poopypants.
Later, George and Harold use the now tiny Goosy Grow to save their lives when they fall to their doom. Making a paper airplane, they enlarge it so that they can fly in it. They later enlarge Captain Underpants with it so that he can beat up Professor Poopypants.
When Evil George, Evil Harold, and Captain Blunderpants get teleported to an alternative universe (ours) via the time machine, they sneak into their twins' tree house. They then find the Goosy Grow and use it to transform Sulu the Bionic Hamster (now a slave because of a 3D-Hypno Ring) into a terrifying monster to help take over the planet.