"Gundam Build Fighters: GM's Counterattack" is a three-episode OVA series that serves as a sequel to "Gundam Build Fighters." The story is set in a world where Gunpla (Gundam plastic models) battles are a popular competitive sport, utilizing advanced technology to bring the models to life. The narrative centers on the aftermath of the 7th Gunpla Battle World Championship, where the Gunpla Mafia, a shadowy organization, attempts to disrupt the peaceful world of Gunpla battles.

The protagonist, Sei Iori, a skilled Gunpla builder, and his partner, Reiji, a talented fighter, are drawn into the conflict when the Gunpla Mafia targets the Gunpla Association. The organization seeks to exploit the Plavsky Particle system, the energy source that powers Gunpla battles, for their own gain. The Gunpla Mafia's leader, Kijima, aims to use the system to create chaos and dominate the Gunpla battle scene. To achieve this, they deploy a powerful and mysterious Gunpla known as the GM Sniper K9, which poses a significant threat to the protagonists and the Gunpla Association.

Sei and Reiji, along with their friends, including China Kousaka and Ral, must confront the Gunpla Mafia and protect the integrity of Gunpla battles. The story highlights their efforts to outmaneuver the antagonists, utilizing their skills in Gunpla building and combat. The narrative also explores themes of teamwork, perseverance, and the importance of fair play in competitive sports.

The OVA introduces new characters, such as the enigmatic Meijin Kawaguchi III, who plays a pivotal role in the conflict. The series maintains a balance between intense Gunpla battles and character development, providing a cohesive continuation of the original series' storyline. The resolution of the conflict reinforces the values of creativity and sportsmanship that are central to the "Gundam Build Fighters" franchise.