You go inside, and get the advancement, but not teleported,
Also in the end you spawn at y=16, that you will have to climb up 200 blocks, where the obsidian things are underground, also the enderdragon has tracking issues. He can't find the Crystals and doesn't regenerate, Flys like a tunneler, doesn't destroy something, the endercrystals are spawned below the portal
The gateway generated underground, but did teleport me to the outer end islands when I threw an ender pearl in.