If you're looking for manga similar to Mobile Fighter G Gundam, you might like these titles.
Duke Fleed is the sole survivor of an attack on the Vegan Empire; he defends his second home, the Earth, from the Vegan attack, together with the giant robot Grendizer.
Saiga Junki is a normal high school boy who tends to be a little apathetic in general but who does manage to get better than average grades. When his archaeologist father, who went to investigate some ruins, is found dead, the family travels to the excavation site where Junki meets a mysterious stranger who gives him a ‘gift’: an ancient artifact that his father left for him…
The greatly weakened Earth Federation comes under attack by the BESPA forces of the spaceborn Zanscare Empire; only the AEUG-like League Militaire stands in their way. The mass-produced Victory Gundam is the League Militaire's secret weapon against the invading BESPA.
Oldrin, who disappeared a while before, now attends Pendleton School of Madrid. And Orpheus, now Liar, is appointed as the Head Knight of the Glinda Knights.
The Calamity War that engulfed all of Earth, and the Gundams deployed then which were said to have brought it to an end. 300 years later, some have been hidden, some abandoned. Argie Mirage, a boy who carries the past of his family's death at the hands of a Gundam, meets Volco Warren, a boy who wishes to rebuild the Gundam his family left behind.To destroy a Gundam. To rebuild a Gundam. In the midst of the two boys' intersecting fates, the Astaroth will awaken from its 300 year slumber.
300 years after a great conflict between Earth and Mars known as the "Calamity War," a woman named Cordelia sets out on a journey to Earth to speak for the independence of the Martian city of Chryse, which is under the control of the Earth government. Escorting her is the private security company CGS members Mikazuki Augus and Orga Itsuka. When a group named Gjallarhorn attacks CGS and Cordelia, Orga sees this as a chance to rebel against CGS and launch a coup. Mikazuki and Orga are thrust into a new conflict. To fend off Gjallarhorn, Mikazuki rides an old mobile suit from the Calamity War, powered by a nuclear reactor, the Gundam Barbatos.
This manga series focuses on Fereshte, an autonomous branch of Celestial Being that is also in possession of several previous generation Gundams.
During a contest to win rights to salvage and perform shows in the Sea of Lorelei, Rick Aller rescues Rosa the Second (Rosa II) when her mercenary crew turn on her. With her mobile suit damaged, and his own small mobile armor damaged as well, they hide inside a sunken submarine that has been resting on the ocean floor since the 7th Space War. While investigating the ship they discover a black GX-9900 Gundam X and decide to use the mobile suit to escape in. But when they open the cockpit they discover the pilot of the mobile suit is still inside of it and in a state of suspended animation. Awakening him they make their escape from the ship and defeat the mercenaries that turned on Rosa II.