The Dragon's Gate is a location in Fire Emblem: Rekka no Ken that was used by the dragons to retreat from Elibe after the Scouring. The gate is located on Valor, The Dread Isle. In the storyline of Rekka no Ken, the main antagonist, Nergal, attempts to open the Dragon's Gate and seeks to control all the dragons that come through. Nergal also uses it as a primary base for himself and his Morphs.
Not much is known about the realm beyond the Dragon's Gate, except that it is where Ninian and Nils are from, and is free from the effects of the Ending Winter, allowing Dragons to maintain their natural form.
The Dragon's Gate can only be opened by dragons, which is why Nergal needed Ninian or Nils to open it for him. Despite not being able to open it, Nergal was some how capable of talking to Ninian and Nils through the gate, and convincing them to open it from their side. After the defeat of the Fire Dragon, Nils (and Ninian if she does not have an A support with Eliwood), will go back through the gate to their world.