Although he has never spoken a single word of English in the series, he was voiced by Hank Azaria.
Gerald and Maggie were born on the same day. As the service in Springfield hospital is always horrible, there was only one diaper left and it was given to Maggie. Gerald was wrapped in the discount section of the local newspapers, developing Gerald a terrible rash which has never disappeared. Since then, Gerald blames Maggie for everything bad that has happened to him.
Once they meet each other, Maggie and Gerald angrily leer at each other. It usually never get further than to bad looks; however, the two babies were seen fighting on St. Patrick's Day. They also shake their fists at each other in the supermarket during the new Simpsons sequence. Homer often mistakes Maggie for Gerald.
It also looks like Gerald is not a very mischievous and uncooperative child only when he is with Maggie. He was once rescued by Bart's new dog Laddie from an unspecified disaster. As Mayor Quimby later told Gerald, "Baby Gerald, we can't help but wonder what mischief you'll get into next.".
When Marge started a daycare center, Gerald was one of the babies that she was looking after. During the day, Gerald blamed Maggie for everything he had done, including thing pencils on Marge, spilling Maggie's milk and breaking the lamp. As Marge who didn't suspect anything was rebuking Maggie, Gerald offered to help with cleaning the mess and therefore he got a special reward from Marge. His bullying eventually led unhappy Maggie to run away from home.
According to some future episodes, Maggie and Gerald stopped being enemies, got married and even had a baby together.
The Longest Daycare
Gerald was the main antagonist for the short The Longest Daycare. His goal is to smash up every butterfly he sees and add it to his collection by drawing a square around every one he smashes. He tries to go after Maggie when she finds a butterfly popping out of a chrysalis (after finding out that's what caterpillars become). His path is continually blocked, but when Maggie sets the butterfly free, Gerald uses the blinds to smash it, much to Maggie's dismay. However, it is revealed, to his dismay, that it is actually Maggie's bow, which was switched with the butterfly.