Developed by the UN Forces, the GN-XII is the successor of the GN-X. In the GN-X series, newly developed machines are not completely brand new, but retrofits of existing units. Thus, the GN-XII units are mostly created by upgrading existing GN-X units through parts swapping. The GN-XII also incorporated data from the GNX-604T Advanced GN-X, and has hardpoints on its shoulders and waists for mounting weapons. Another feature of the GN-XII is that the GN Particle Generators on its chest and waist, which are for controlling GN Particles, have been miniaturized compared to those of the GN-X.
Using the operating data from Team Trinity's Gundam Thrones as a reference, variations of the GN-XII with different equipment were created for a variety of missions and situations. Despite their differences, the GN-XII and its variants can work cooperatively in battles. For example, the GN-XII and GNX-607T/BW GN-XII Cannon can shoot from close and long range respectively at the enemy, whose movement is restricted by the close combat specialized GNX-607T/AC GN-XII Sword. The GN-XII is later upgraded into the GNX-609T GN-XIII, which retains the hardpoints system.