The Goosy-Grow 4000 was one of two inventions that Professor Poopypants invented to share to the world. However he never got a chance to share either of his inventions due to everybody laughing at his silly name. When he went insane due to George and Harold writing a comic book about him, he used this invention to make his Gerbil-Jogger 2000 (which was created while he was the science teacher at Jerome Horwitz Elementary School) bigger. He also used it to enlarge a pencil that Ingrid Ashley gave him.
When Captain Underpants was instructed to get this invention to make the school grow back to normal, the Professor zapped him and the Goosy-Grow with the Shrinky-Pig 2000 making both smaller. The now small invention landed in George's hand, but he figured out how it works and what he and Harold must do. They are thrown off the roof of the school by the Professor when they to try to get the school back to its proper height. Harold quickly made a paper airplane which George enlarged with the Goosy-Grow 4000. Later they tried to make the Captain bigger so he can fight the Professor. Captain Underpants landed on Harold's finger, but when George aimed the Goosy-Grow at the Captain, he accidentally zapped Harold's right hand, making it bigger. George then zapped Captain Underpants a few times til he was about the same height as the big Gerbil-Jogger 2000
After Captain Underpants defeated Professor Poopypants. Harold used the Goosy-Grow 4000 to enlarge himself and George before they put the school back where it was and enlarged it with the Goosy-Grow 4000 while Harold used the Shrinky-Pig 2000 to shrink Captain back to his proper height. George and Harold then kept both the Goosy-Grow and the Shrinky-Pig with them after Professor Poopypants was arrested.
Later in the 8th book, Evil George stumbled upon this invention in Good George and Good Harold's treehoue and tested it out on Evil Sulu (Sulu hypnotized to be evil by the 3-D Hypno Ring). It then made Evil Sulu grow to the size of a sheepdog. Evil George then gave a few more zaps to Evil Sulu until he was about the size of a giant monster. It is not known if Evil George, Evil Harold, and Captain Blunderpants still have this invention after they used it on Evil Sulu. However it is possibly still in Good George and Good Harold's treehouse, as the evil trio most likely had no further use for it after enlarging Evil Sulu since they only needed it to make Evil Sulu gigantic, so that way he could be used as a distraction while they robbed Frank's Bank.
"Slightly Yonger Tiny Tippy," & "Tiny Tippy," find The "Goosy-Grow 4000." Later, Slightly Younger Tiny Tippy uses the Goosy-Grow 4000 to enlarge himself, so he could save George, Harold, & their Cave People friends.