MAJOR SPOILERS ahead for Crisis on Infinite Earths Parts 4 & 5. The Arrowverse’s annual crossover event, Crisis on Infinite Earths, concluded with the Legends of Tomorrow episode “Part 5,” which ended with a highly-unusual Easter egg: the introduction of Gleek. The episode’s final shot was a closeup of an empty Star Labs cage (and a fresh banana peel) with the label “subject name: GLEEK.”

https://screenrant.com/crisis-infinite-earths-gleek-explained-dc-comics/