The first Earth Federation mobile suit developed under their top-secret "Project V", the RX-75 Guntank was hardly a mobile suit at all. The RX-75 project was initiated four years before the start of the war, based on little more than rumors of Zeon's new weapons. The Guntank was nothing more than a fusion of a tank with the upper torso and arms of a mobile suit, and was originally intended to serve as an anti-MS unit when rumors of Zeon's first mobile suits reached the Federation. It later evolved into a crude mobile suit armed with recoilless cannons and missile launchers, and equipped with caterpillar treads instead of legs and packed a serious punch as a heavy fire support and artillery unit.

The RX-75-4, the fourth iteration of the design, features the same Core Block System used in the other Operation V prototypes. However, it still requires two operators - a driver in the Core Block cockpit and a gunner in the head. The Guntank stationed aboard the White Base, the only survivor of the Zeon attack on Side 7, was later modified so that pilot Hayato Kobayashi could operate it singlehandedly from the head cockpit.

Source: Gundamproject.com, HG-UC kit manual