The Abyss is a transformable unit specialized in underwater combat which can transform into a streamlined submersible mode for increased speed underwater. Loosely developed based on the GAT-X252 Forbidden and the GAT-707E Forbidden Vortex, it is designed to operate equally well in space as well as underwater, though unlike the original Forbidden it is incapable of atmospheric aerial combat.
The suit is heavily armed and features a large number of beam and phonon maser weapons. It also features several CIWS guns for anti-MS/missile interceptions as well as torpedo launchers, a set of shell-firing cannons, and a beam lance for close-quarters combat. When operating on the ground or in space, the Abyss's variety of beam weapons allows it to function as a long range support unit.
A set of light machineguns primarily used for shooting down missiles and lightly-armored aircraft. Two are mounted in the Abyss's torso, with a third mounted in its head, usable only in mobile suit mode.
"Scylla" Multi-phase Beam Cannon
A powerful beam cannon of the same model as that used by the Aegis and Calamity Gundams, mounted in the Abyss's torso.
MJ-1602 Beam Cannon x2
A pair of beam cannons mounted to the Abyss's back, which pivot over the shoulders to fire and can be used in mobile armor mode.
MMI-TT101 Mk9 High-speed Guided Torpedo tube x4
Four torpedo tubes, mounted on the shoulder-mounted shields of the Abyss. In mobile suit mode the shields can swing upwards to fire the torpedos at a target in front of the suit. In mobile armor mode the launchers are facing forward.
M68 Dual Cannon x2
Two pairs of shell-firing cannons, mounted on the shoulder-mounted shields
MX223E Triple Phonon Maser x2
Three underwater-use beam weapons mounted inside the shoulder-mounted shields, usable only in mobile suit mode.
MX-RQB516 Beam Lance
Since the Abyss was design for aquatic combat, it lacks beam sabers, equipping instead a beam lance. This lance mounts a physical blade in addition to its beam blade, enabling its use underwater.
The Abyss is equipped with Phase-Shift Armor. Improvements in the technology in recent years mean that it is far more energy-efficient than it previously was, causing a much-reduced drain on the battery.
Like the Forbidden Vortex, the Abyss has a thickly-armored pressure hull around its cockpit block, protecting the pilot in the event the unit shuts down due to power loss or damage.
When the First Junius War ended, more than half of the Earth Alliance's military strength had been annihilated. In the following years, the Atlantic Federation began to work towards rebuilding its lost military might, and they contract this out to two of the Alliance's largest weapons manufacturers - National Defense Industries (formerly known as the Azrael Conglomerate), and Adukarv Mechano-Industries. Over the intervening years, a number of mobile suits are developed, such as the GAT-04 Windam. In addition to this, the two manufacturers begin work on more powerful prototype units. After producing the TSX-MA24F Chaosin conjunction with rival Adukarv, NDI chose to turn its attention to variable mobile suits, and in CE 74 began development on two prototypes: The aquatic mobile suit the high-mobility ground combat GAT-X388 Gaia Gundam and the aquatic assault Abyss Gundam.
The Abyss Gundam was created using the mass-production underwater mobile suit GAT-707E Forbidden Vortex as well as its predecessor, the GAT-X252 Forbidden. Like the Forbidden Vortex, its cockpit block was designed with a reinforced titanium pressure hull to protect the cockpit in the event the mobile suit's own structural integrity should be compromised. However, the Abyss does not use the Forbidden's Geschmeidig Panzer technology, relying instead on its Phase-Shift armor to counteract water pressure. The sensors that were once located in the Forbidden's tail have also been integrated into the unit's main body, making them less vulnerable to attack. Unlike the Forbidden Vortex, the Abyss is designed to be able to operate equally well in space, thus combining the capabilities of the underwater-use Vortex with its predecessor. However unlike the Forbidden, it is not capable of aerial combat on Earth. Its armament reflects this, as in addition to an array of phonon masers, two powerful pairs of shell-firing cannons, and a set of torpedo tubes, the Abyss is armed with a pair of powerful back-mounted beam cannons and a multiphase plasma beam cannon based on that of the GAT-X303 Aegis. For close combat in mobile suit mode it is armed with a lance, which can be used underwater thanks to its physical blade as well as outside the water, in which case it can generate a beam for additional power.
The Abyss is assigned to Extended Auel Neider and initially based on the stealth battleship Nana Buluku, from which it is first deployed on February 14, CE 75, against ZAFT's new warship Minerva, during an operation carried out by rogue extremist Coordinators to drop the remains of Junius Seven onto Earth. Misunderstanding the situation, Auel and his teammates both the terrorists and the Minerva's mobile suits, which were attempting to both stop the colony from falling and protect the multinational Memorial Delegation which was at the site for the anniversary of the Bloody Valentine Tragedy.
The Abyss was later transferred to Earth and operated from the Atlantic Federation carrier John Paul Jones. During its time on Earth, it again battled against the Minerva on a number of occasions, first in the Indian Ocean, where it succeeded in sinking one of the Minerva's two escort submarines in the Indian Ocean, though it was prevented from attacking the second by ZAFT mobile suits including the new UMF/S-6 ASSH mobile suits, whose high speed and mobility - if lower than the Abyss's own - and numerical advantage made them dangerous foes.
The Abyss would face the ASSH on two additional occasions, first when they attack the Minerva in the Dardanelles, then a second time off the coast of Crete. In this second engagement, it would meet its end in battle against the ZGMF-X56S/γ Blast Impulse Gundam, which would destroy the Abyss by impaling it with a thrown beam javelin, which pierced the Abyss's cockpit, killing its pilot.