The Gate is a giant door in Truth's domain, that is one of the most important factors in the entire Fullmetal Alchemist series. The inside of this door is said to be God's domain, and it is supposedly where all unknown alchemical knowledge in existence is held. All Alchemists that attempt Human Transmutation are taken to Truth's domain and then dragged through the Gate, where they are given all Alchemical knowledge possible, depending on what was given to enter the Gate.
Related ThreadsShould Bill Gates compensate Hollywood? - last post by @ Jan 7, 2008
Within Our Gates (1920) - last post by @ Feb 28, 2018
Gates of Fire - last post by @ Mar 29, 2004