Gundam True Odyssey: Ushinawareta G no Densetsu is an RPG inspired by the Gundam series anime. Far in the future a machine called the "G System" is created which allows energy and materials to be turned into objects. Shortly after civilization is nearly destroyed by the machine, an event called The Great Fall. The games story begins after The Great Fall; what's left of mankind has begun to rebuild and stabilize a new society. One day a mysterious mobile suit arrives and initiates some random attacks on various towns, including an orphanage which is destroyed leaving only two survivors, Tristan and Fritz. These two survivors will then embark on a mission to take revenge for the destruction of their orphanage, and along the way will learn the mobile suit responsible for the destruction is part of even bigger plot to take over the world.

At the beginning of the game you control Tristan and Fritz; as the story progresses, other characters will both join and leave your party. Each of the characters in your party has a mobile suit which is used to fight in battles as well as explore the vast regions of the world. You will also come across various towns where the characters can leave the mobile suits; in the towns you can talk with people, visit shops to buy items, rest to regain health and save the game, as well as learn about new missions to undertake. In most areas except towns (such as exploring the lands or in dungeons) you will encounter battles in both random locations and set locations; the set location battles are indicated by glowing crystals on the screen and are usually located strategically blocking an area you need to get to or to advance the story.

The game uses a turn based battle system. Different types of attacks are available; hand to hand attacks tend to be more powerful but leave you open to a counter attack while ranged attacks are safer but deal less damage. On each turn each member of your party is given a limited amount of energy to use for attacks; different attacks use different amounts of energy. Lower powered attacks typically use less than the energy quota for a turn allowing you to save up the unused energy for a more powerful attack in a later turn. Each character also has a limited number of technique points which can be used for additional abilities such as healing, increasing armor, special attacks, and so on. At any given time up to three characters can be in the front line and take action in battles; any additional characters remain in the back line and are unable to attack, however you can switch characters around on each turn. As you win battles, experience points are earned allowing your characters to level up and become more powerful.