Not initially taking the threat of "mobile suits" seriously, the Earth Federation was a latecomer during the One Year War when it came to development and production of its own mobile suits to counter the Zeon Duchy. Based on combat data from the RX-78-2 Gundam's "learning computer" system and experimental GM-type units, the Federation's first true mass-production unit was simply named the RGM-79 GM. Though the first GM units weren't available until UC November 0079, the Federation's Jaburo and Luna II factories quickly made up for this late start, fielding enough units in time to retake contested areas on Earth and to participate in the Federation's Operation Star One offensive on Zeon's space front.

The RGM-79 is the Earth Federation's first true mass-production mobile suit, developed using test data from the prototype RX-78 Gundam and the earlier incarnations of the GM. This standard GM is a simplified version of the Gundam, eliminating many features to reduce cost and complexity. The Gundam's durable Luna Titanium armor is replaced with mundane titanium alloy, one beam saber eliminated, and the Core Fighter replaced with a non-ejectable core block. Instead of a beam rifle, the GM is armed with a hand-held machine gun or a short-range beam spray gun. With the Federal Forces already on the offensive, the introduction of the GM units to the front lines assured the Federation's victory and descendants of the GM line would continue to serve the Federation military for the next fifteen years.

Source: Gundamproject.com, HG-UC kit manual