Mobile Suit Gundam 00, Mobile Suit Gundam: Iron-Blooded Orphans Recommendations
G-Tekketsu has the concept of child soldiers being used as a very prominent plot point. This idea has been touched upon since the very first Gundam series but season 1 of 00 was the first to use it prominently and openly.
In 00, the protagonist used to be a child soldier, part of a religious fundamentalist group called KPSA, and is now part of a different group called Celestial Being who use armed force to pursue a world without war and therefore a world where children do not have to be soldiers like he was.
In G-Tekketsu, the main characters are all child soldiers/slaves
Being a part of the same franchise, you can obviously expect a degree of similarity within all Gundam titles. 00 bears most similarity to IBO so far as the characters both possess and are not objected to using the mobile suits right off the bat. some thematic similarities lies within the morality of war and the subject of child soldiers and their exploitation by various religious sects and corporations. the central groups are both militaristic, and respond and act appropriately as you'd expect of such a group, professionally, practically and efficiently.