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.
When truth announces who he is he is one or all or also known as god the universe or truth behind the door is nothing but not nothing it is what you think it is a fragment of your missing mind... Presented in one of the episodes the door leads to many things presented in a cross over of Hitler and the Nazi war Edward Elric entered the portal or gate way what ever you wanna call it and entered our world when the Nazi War happened. Regarding the gate way to truth that is truth the gate way takes different forms if you ever followed the story correctly it specifically states that Gate Way is a kinda door or portal to what ever your dreams can imagine. The professor from the Germany side of the door talks among parallel worlds that are connected invisibally and can't be seen and in different dimensions there is another type of you that has it's own personality. Clearly though you can only do this by being in the Full Metal Alchemist world specifically the gate way can lead to many places. ^^' That's why there is no point to the universe because everything is infinite just read the Manga and watch the show and decode what it is trying to say... ALSO An edit even though you have different yous in different dimensions supposedly the real you is in the Full Metal Alchemist World since other dimensions logically don't exist... Who ever you are is not what you are but what you are is in the FMA world... Truthfully the show talks if you play god it back fires but also we already know everything because the answer to everything is clearly NOTHING.
The Gateway of Truth is the meeting point of infinity, which was theorized when the being inside of it describes himself as God, The Truth, and yourself. Edward comes to discover that the gateway you see is actually your perception of the truth, explaining why Alphonse's and Izumi's gates look different. You can also see all of Edward's memories when he was inside the truth. However in the original Fullmetal Alchemist anime, the gateway is access to all universal knowledge, explaining why you can see John F. Kennedy and moments from World War II.