The Green Door appears to have been created by a unknown creator to imprison the One-Below-All within the Below-Place, a place within Hell that was lower and much harder to access than most other dimensions. Many individuals do not even know that the Green Door exists, let alone what resides within it. When a person infused with Gamma radiation dies, their soul passes through the Green Door itself, and they usually see it as a flash of green light in the distinct shape of a door, hence the name. After passing through it, the soul of that person becomes apart of the Below-Place, and are transformed into a physical husk, enslaved by the One-Below-All and are reduced to repeating quotes and catchphrases of their former life, or are turned into hideous creatures.
However, when a person from the Below-Place is brought back to life by any means, their memory of it and the Green Door are usually stripped away and removed. This may be due to the nature of the dimension itself, or the One-Below-All's influence. The only exception to this is when one is forcefully brought into the Green Door alive, not dead.
The Green Door was first partly broken open when the first Gamma Bomb was detonated due to the amount of gamma energy that was unleashed. This was also the Gamma Bomb that created the Hulk within Bruce Banner. The Green Door can be completely opened by a massive influx of Gamma radiation within the area where the Gamma Bomb went off, as showcased when the Absorbing Man, having drained most of Hulk's Gamma radiation (to the point that he was reduced to a extremely skinny size) due to a Bannerman Gene Enhancement Package was able to implode at the exact spot where the bomb went off, breaking the Green Door open and allowing the One-Below-All to transform all of Mexico into the Below-Place and enter Earth. However, if the source of the Gamma radiation being used to keep open the Green Door is interrupted in any way, it can be closed once more.
While the Green Door is closed, the One-Below-All is forced to use its powers to control and manipulate those mutated with Gamma radiation, usually in a attempt to find stronger hosts, or to open the Green Door and set itself free. However, considering Brian Banner (a individual with no exposure to gamma radiation) was effected and even viewed the Green Door for a split second, this suggests that the One-Below-All can influence normal people as well through the Green Door, through perhaps to a much lower extent.
The Green Door also looks different depending on where you view it: from outside of the Below-Place, it appears as a large rectangular shape or a series of green swirls forming together around a bright abstract object, while within the Below-Place itself it becomes a massive ray of green light coming from the sky and appearing from the highest mountain there.