The GX-9901-DX Gundam Double X (aka Gundam DX, Double X, DX) is a mobile suit produced by the New United Nations Earth in After War Gundam X and piloted by Garrod Ran.

The GX-9901-DX Gundam DX was created by mobile suit engineers of the newly formed New United Nations Earth (NUNE). NUNE recovered the wreckage of GX-9900 Gundam X used by Jamil Neate during the 7th Space War and transplanted some of its system to the new mobile suit dubbed Double X. Furthermore, as creating a completely brand new design was extremely difficult, the mechanics used the Gundam X as the base unit when developing the Double X.

The Gundam Double X has an improved Satellite System, the "Satellite System Mk-II". It consisted of a larger reflector unit on its back with six panels, and several energy radiator plates on the limbs as part of the new improved cooling system to increase the microwave energy absorption. Accompanying this increase in energy storage capacity, the single-barreled Satellite Cannon was upgraded into the two-barreled Twin Satellite Cannon, allowing Double X to unleash destructive force several times that of the Gundam X. The power of the Twin Satellite Cannon was demonstrated when Garrod Ran used it to destroy the island of Zonder Epta in one shot, and shown again when used to destroy the Space Revolutionary Army's Colony Laser. As the old system transplanted from Jamil's Gundam X still contains his brain wave patterns, Double X can use the Twin Satellite Cannon without a Newtype as its pilot.

The Gundam Double X is also equipped with two Hyper Beam Swords stored on its side skirt armor, two Head Vulcans, four Breast Launchers and can use custom-made weapons including the Buster Rifle and Defense Plate. Because of its similarities to the Gundam X, the Gundam Double X can dock with a support craft developed for the Gundam X called the GS-9900 G-Falcon. While docked with the G-Falcon, the Double X is unable to use some of its weapons, but has the ability to use the weapons equipped on the aft section of the G-Falcon. Also, while joined with the G-Falcon, the Gundam Double X is able to safely re-enter Earth's atmosphere.

Like the Gundam X, the Double X had a security key in the form of a G-Controller that is needed to activate and control it. However, while the Double X has its own G-Controller, its base system is still the same as the Gundam X. This means that even without the Double X's G-Controller, it can still be operated by anyone who has an older-model G-Controller used with the Gundam X - as demonstrated by Garrod, who stole the Double X in this manner.