What is inside the Gate of Truth? In FMA and FMA: Brotherhood, the Gate of Truth is shown to have an eye and a bunch of arms inside of it. The arms drag people into the gate during human transformation. I know that the creature sitting in front of the Gate is Truth, but what is inside of the gate (i.e. the arms and the eye)?
In the manga and Brotherhood, there are two things within the Gate of Truth: God, and all alchemical knowledge that has ever existed or will ever exist. The Fullmetal Alchemist Wiki says it best:


  It is the source of alchemical knowledge and the entrance to God's domain, where no mortals are meant to tread.


As is stated in this answer, this is where mortals end up when they attempt human transmutation. They pay a toll, and are granted access to all alchemical knowledge within.



While an alchemist is within this realm (the realm of non-reality), they will inevitably learn something about alchemy, and in fact anyone in the series who recalls this information gains the ability to perform transmutation without a circle.

In the 2003 anime, there are some differences. Notably, the realm itself is the source of alchemical energy (rather than knowledge), and the Gate acts as a portal between that energy and the FMA universe. You can read more about the differences on the FMA Wiki.