The show within the show, or anime series within anime series in Martian Successor Nadesico
Based on other super robot shows such as Mazinger and Getter Robo, Gekiganger was an anime series that existed within Martian Successor Nadesico. Clips from the series was shown from time to time to encourage Akito Tenkawa to do something crazy or heroic. In beginning of Nadesico series, Gekiganger was more of joke, or humor but as series went on, it became something important to drive the main story line.
The series actually ended up having its own OVA later.