The Gate

The gate of alchemy, more commonly referred to as the Gate or the Doors of Truth, is a structure that appears within both the anime and manga. It is the source of alchemical knowledge.

In the anime
The gate is a double door gate. It links the real world (Earth) and the fake world (Asteris) together. It is featured in some of the episodes and is in the movie, Conqueror of Shambala. There are many different forms of the gate. In the movie it takes the form of a portal and the regular gate. It opens when a homunculus is transmuted. This happens in the Conqueror of Shambala, when Alphonse Elric transmutes Wrath to make a permanent portal or gate. On the other side, the portal is opened when Van Hohenheim is transmuted to make the portal.

In the manga
In the manga, the Gate can only be reached through performing human transmutation. It is seen as a floating pair of stone doors, with a design of the Sephirot on it. A humanlike figure named "Truth" sits in front of the doors, and converses with the alchemist that has attempted human transmutation.

The doors open into a black void, with a multitude of shadowy hands reaching ot from it and a stylized eye in the centre.

Truth is always seen with the body parts that the alchemist has lost from human transmutation on his own body, implying Truth appears different for each alchemist, though we only ever see Ed's version of it.

The homunculus Gluttony was a failed experiment by Father to try and recreate the Gate. Gluttony's true form has fis ribs opening up and forming a void, with an eye in the centre almost identical to the one in the Doors of Truth. It does not open into the inside of the doors; instead, it's the space in which what Gluttony swallows is contained.