The End Gateway is a portal introduced in the 1.9 Combat Update.
This portal spawns after the Ender Dragon is killed in The End; up to twenty End Gateways can be generated by respawning and rekilling the dragon. It leads to the outer islands of the End, which are a thousand blocks away from the central island, where End Cities and Chorus Plants generate. If a player has previously killed the Ender Dragon before the 1.9 update, they must respawn the dragon and then kill it again in order for the gateway to appear.
Throughout the End, more end gateways will generate. These aid a player in getting back to the main island: when entered, a player is taken back to the obsidian platform where they spawned, although it will not trigger the reformation of the platform.