Gary Stu only appears in Yu-Gi-Oh: The Other Abridged Movie. He doesn't like card games, much to just about everyone's horror. He claims that he just doesn't see the appeal in it and would rather study or something. He gets irritated because nobody listens to him.
He is said to be Yugi's "cool" friend even though both Yugi and Gary admit they've never met before. Later Yugi considers him a wuss and Joey threatens to beat him up if he doesn't watch the duel between Yugi and Kaiba. He changes his mind about card games after watching Yugi and Kaiba's three-hour long duel.
He was later sent by his parents to an island and ordered to kill all his classmates in a reference to Battle Royale. Gary managed to return, only to be devoured by Godzilla.
His name is a reference to the male version of the character archetype Mary Sue, with his status of "Yugi's best friend" as a reference to characters created in fanfiction.