The source of all souls in the FMA universe. Behind it lies the "truth" It is also the source of alchemic power in the FMA anime
The source of all life itself is The Gate. In the beginning of the series, Ed and Al both see The Gate. Ed watches helplessly as The Gate takes Alphonse's entire body as the cost for Human Transmutation. The Gate then immediately takes Edward's left leg to complete the transmutation. Before coming back to his home, Edward is forced by The Gate itself to view the Truth that lies within it. After which Edward acquires a higher understanding of alchemy and realizes that if he acts now he might be able to get Al back. And with that Ed is able to fix Al's soul into a suit of armor by placing an alchemic symbol on the armor and then placing the same symbol over his heart and both arms. This is something Ed could not have possibly known unless he saw what lied within The Gate. Willing to get back his brother he knocks on The Gates doors, begging for his brother. The Gate reopens, but this time taking his right arm as payment, and with that Alphonse's soul is tied to the armor. While it is true that the Truth exist inside The Gate so do the souls of all who have died, they are what grants The Gate its power over knowledge and the Truth. They are also the ones that collect the payment made when Human Transmutation is performed. However what is revealed near the end of the series is what lies on the other side of The Gate isn't heaven, hell, or oblivion, no what truly awaits on the other side is a paralel world. Our world, where alchemy isnt possible, but Newtons laws of physics exist. An even more shocking truth is that the lives of all that die on that side of gate become the energy used to make transmutations in the FMA universe. This is possible since every person holds a small piece of The Gate and the Truth within them but as they grow older and become more attatched to this world their bond with The Gate weakens. But by using someone whos bond with The Gate is still strong like a baby one can summon The Gate without the use of Human Transmutation. All the Homunculi seem to hold a unique tie to The Gate as when Dante opened The Gate, Wrath went into the room, screaming and begging for her to close The Gate. In addition to that Homunculi tend to age while inside The Gate, as well as healing their inperfect bodies that were transmutated. Shown how Wrath looked disfigured when created but after escaping The Gate he looked like an ordinary child. also all Homunculi are Catalyst as they hold a much greater connection between The Gate and themselves, then humans do. For that reason Homunculi can be transmutated without any Human Transmutation rebound since Homunculi arent truly human. With this transmutation and the proper transmutation circles The Gate can be opened.
However in the manga and second anime, the Gate seems to not have any link to our world or the souls of those in it.