The Gate Children

The Gate Children are entities that make up the collective being that exists inside the Gate in the 2003 anime series storyline. Shadowy and childlike, these primordial beings appear to have a strong connection to the Gate's function, serving as the owners of the umbral, prehensile limbs that emerge to deconstruct things and people that seek to pass through the Gate of Alchemy itself. Living between the two gates of the parallel worlds, Earth and Amestris, it is highly possible that they are humans who have died in infancy or before birth, based on the fact that one of these Gate Children was recorded to have escaped the Gate and reached Amestris as the Homunculus Wrath, relating upon his rebirth memories of his birth-mother Izumi Curtis.

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 affcted 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 in tact, save for the mortal wound. 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.

Trivia

 * 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.