wikipedia:While Project V was underway, the Federation also began production of experimental units using spare parts originally intended for the RX-78 Gundam. Using these parts they built approximately twenty specialized ground combat-model christened the RX-79[G] Gundam Ground Type during the mid-part UC0079.
Though in many ways similar to its RX-78-2 predecessor, the RX-79[G] was quite a bit different in other ways, particularly in its armament. The beam sabers were stored in the legs, as the backpack mounted an equipment rack to carry storage containers, parachute packs, or other field options. The head vulcan guns were removed, as the head now carried a periscope for underwater and trench warfare. Although sporting a smaller generator than the RX-78, the Ground Type Gundam still had the generator power to use a beam rifle, though it was often equipped with a standard 100mm machinegun or a heavy 180mm cannon. A single vulcan gun and a multiple-use "multi-launcher" mounted in the torso's left breast rounded out the new armament. In order to make use of space, two extra ammunition packs for the machinegun were placed on the sideskirt armor.
As the RX-79[G] was built out of spare parts from Project V, replacements parts were very limited. Luckily the RGM-79[G] shared 80% of the same parts as the Gundam, making field repairs easier but sometimes resulted in a few customized mobile suits. Source:http://gundam.wikia.com