It is not explained what the Gate Children are, but there are a few possibilities. Since infants are said to be "closer" to the Gate, they could be souls of humans who had yet to be born or died during birth. A specter of Izumi Curtis also appeared in the Gate after both she and Wrath died, so they could be dead souls, but it is worth noting that Izumi appeared fully human. It is impossible to verify any of these, because the anime does not specify what happens to the soul before death or after, save that it becomes energy. Another possibility is that they are humans who have become trapped in the Gate. This possibility was semi-confirmed with the Homunculus Wrath, who was given back to the Gate by his mother, Izumi, after she attempted human transmutation. Though Wrath became a Gate Child after this event, it is unclear if this is what would happen to anyone who was sent inside of the Gate but failed to pass through, or if this is the origin of all Gate Children.As depicted in Fullmetal Alchemist the Movie: Conqueror of Shamballa, humans that pass into the Gate through artificial means become exposed to the Gate Children for an unnatural length of time and begin to meld with them. This mutation appeared to take two distinct forms.
In one, the soldiers who passed through the Gate simply died. Though they continued to move and attack, opening their armor revealed that they had already been crushed, probably by the Gate Children themselves. However, they could be killed a second time, implying that their "reanimation" was made possible because they had been implanted with the soul of a Gate Child.
The second type is seen only with Dietlinde Eckhart, who received great Alchemical power, likely due to the Gate Children's existence as part of the Gate itself. Even though alchemy is impossible in her home world, Eckhart had theoretical knowledge of it, which might explain why the Gate affected her differently.
Additionally, it seems that the greater the person's exposure, the more Gate Children become attached. Eckhart herself had many more attached to her than the soldiers. After passing through the Gate for a second time, she no longer resembled a human being, instead appearing as a ten-to-twelve foot monstrosity with red eyes. Curiously, the transformations instantly vanished once she was killed, leaving her body perfectly intact, save for the mortal wound. Since Ed had previously stated that humans were not very malleable through alchemy, it could be inferred that the Gate Children had only minimal attachments to Eckhart's body, serving as extensions, rather than changing her physiological makeup entirely.
The Gate Children also implanted themselves onto the rockets that the Thule Society were using to pass through the Gate. What effect, if any, that this had is unknown.
- Despite being created specifically for the 2003 anime, the Gate Children are invented to explain the shadowy, prehensile arms that appear in the manga storyline when the Gate is opened and when Father and Pride reveal their true forms. Additionally, it is likely that these beings are based somewhat off of the entity Truth from the manga storyline.