The very first G-Series of G3 System is G1 or Kamen Rider G1.

G1 is the earliest model of the Tokyo Metropolitan Police's G-Series program, which was designed to fight the unidentified lifeforms. The G1 model is an armored exosuit that is designed to emulate Unidentified Lifeform #4, even resembling him in appearance. At some point, the prototype was considered a failure and put in storage, but the research data on its development laid the groundwork for future models of G-Series weapons.

However one of the Lords, Hydrozoa Tegula, steals the prototype G1 armor, which was kept in storage along with many prototype weapons designed to combat Unidentified Lifeforms in the police department's basement. He fuses with it, changing the colors of the armor into a scheme similar to that of Kamen Rider Kuuga's Rising Mighty Form, creating Kamen Rider G1.

But in the manga adaptation of Kamen Rider Kuuga, Kaoru Ichijo is the user of G1 system.